diff options
Diffstat (limited to '')
| -rw-r--r-- | Documentation/arch/powerpc/kvm-nested.rst | 634 |
1 files changed, 634 insertions, 0 deletions
diff --git a/Documentation/arch/powerpc/kvm-nested.rst b/Documentation/arch/powerpc/kvm-nested.rst new file mode 100644 index 0000000000..630602a8aa --- /dev/null +++ b/Documentation/arch/powerpc/kvm-nested.rst @@ -0,0 +1,634 @@ +.. SPDX-License-Identifier: GPL-2.0 + +==================================== +Nested KVM on POWER +==================================== + +Introduction +============ + +This document explains how a guest operating system can act as a +hypervisor and run nested guests through the use of hypercalls, if the +hypervisor has implemented them. The terms L0, L1, and L2 are used to +refer to different software entities. L0 is the hypervisor mode entity +that would normally be called the "host" or "hypervisor". L1 is a +guest virtual machine that is directly run under L0 and is initiated +and controlled by L0. L2 is a guest virtual machine that is initiated +and controlled by L1 acting as a hypervisor. + +Existing API +============ + +Linux/KVM has had support for Nesting as an L0 or L1 since 2018 + +The L0 code was added:: + + commit 8e3f5fc1045dc49fd175b978c5457f5f51e7a2ce + Author: Paul Mackerras <paulus@ozlabs.org> + Date: Mon Oct 8 16:31:03 2018 +1100 + KVM: PPC: Book3S HV: Framework and hcall stubs for nested virtualization + +The L1 code was added:: + + commit 360cae313702cdd0b90f82c261a8302fecef030a + Author: Paul Mackerras <paulus@ozlabs.org> + Date: Mon Oct 8 16:31:04 2018 +1100 + KVM: PPC: Book3S HV: Nested guest entry via hypercall + +This API works primarily using a single hcall h_enter_nested(). This +call made by the L1 to tell the L0 to start an L2 vCPU with the given +state. The L0 then starts this L2 and runs until an L2 exit condition +is reached. Once the L2 exits, the state of the L2 is given back to +the L1 by the L0. The full L2 vCPU state is always transferred from +and to L1 when the L2 is run. The L0 doesn't keep any state on the L2 +vCPU (except in the short sequence in the L0 on L1 -> L2 entry and L2 +-> L1 exit). + +The only state kept by the L0 is the partition table. The L1 registers +it's partition table using the h_set_partition_table() hcall. All +other state held by the L0 about the L2s is cached state (such as +shadow page tables). + +The L1 may run any L2 or vCPU without first informing the L0. It +simply starts the vCPU using h_enter_nested(). The creation of L2s and +vCPUs is done implicitly whenever h_enter_nested() is called. + +In this document, we call this existing API the v1 API. + +New PAPR API +=============== + +The new PAPR API changes from the v1 API such that the creating L2 and +associated vCPUs is explicit. In this document, we call this the v2 +API. + +h_enter_nested() is replaced with H_GUEST_VCPU_RUN(). Before this can +be called the L1 must explicitly create the L2 using h_guest_create() +and any associated vCPUs() created with h_guest_create_vCPU(). Getting +and setting vCPU state can also be performed using h_guest_{g|s}et +hcall. + +The basic execution flow is for an L1 to create an L2, run it, and +delete it is: + +- L1 and L0 negotiate capabilities with H_GUEST_{G,S}ET_CAPABILITIES() + (normally at L1 boot time). + +- L1 requests the L0 create an L2 with H_GUEST_CREATE() and receives a token + +- L1 requests the L0 create an L2 vCPU with H_GUEST_CREATE_VCPU() + +- L1 and L0 communicate the vCPU state using the H_GUEST_{G,S}ET() hcall + +- L1 requests the L0 runs the vCPU running H_GUEST_VCPU_RUN() hcall + +- L1 deletes L2 with H_GUEST_DELETE() + +More details of the individual hcalls follows: + +HCALL Details +============= + +This documentation is provided to give an overall understating of the +API. It doesn't aim to provide all the details required to implement +an L1 or L0. Latest version of PAPR can be referred to for more details. + +All these HCALLs are made by the L1 to the L0. + +H_GUEST_GET_CAPABILITIES() +-------------------------- + +This is called to get the capabilities of the L0 nested +hypervisor. This includes capabilities such the CPU versions (eg +POWER9, POWER10) that are supported as L2s:: + + H_GUEST_GET_CAPABILITIES(uint64 flags) + + Parameters: + Input: + flags: Reserved + Output: + R3: Return code + R4: Hypervisor Supported Capabilities bitmap 1 + +H_GUEST_SET_CAPABILITIES() +-------------------------- + +This is called to inform the L0 of the capabilities of the L1 +hypervisor. The set of flags passed here are the same as +H_GUEST_GET_CAPABILITIES() + +Typically, GET will be called first and then SET will be called with a +subset of the flags returned from GET. This process allows the L0 and +L1 to negotiate an agreed set of capabilities:: + + H_GUEST_SET_CAPABILITIES(uint64 flags, + uint64 capabilitiesBitmap1) + Parameters: + Input: + flags: Reserved + capabilitiesBitmap1: Only capabilities advertised through + H_GUEST_GET_CAPABILITIES + Output: + R3: Return code + R4: If R3 = H_P2: The number of invalid bitmaps + R5: If R3 = H_P2: The index of first invalid bitmap + +H_GUEST_CREATE() +---------------- + +This is called to create an L2. A unique ID of the L2 created +(similar to an LPID) is returned, which can be used on subsequent HCALLs to +identify the L2:: + + H_GUEST_CREATE(uint64 flags, + uint64 continueToken); + Parameters: + Input: + flags: Reserved + continueToken: Initial call set to -1. Subsequent calls, + after H_Busy or H_LongBusyOrder has been + returned, value that was returned in R4. + Output: + R3: Return code. Notable: + H_Not_Enough_Resources: Unable to create Guest VCPU due to not + enough Hypervisor memory. See H_GUEST_CREATE_GET_STATE(flags = + takeOwnershipOfVcpuState) + R4: If R3 = H_Busy or_H_LongBusyOrder -> continueToken + +H_GUEST_CREATE_VCPU() +--------------------- + +This is called to create a vCPU associated with an L2. The L2 id +(returned from H_GUEST_CREATE()) should be passed it. Also passed in +is a unique (for this L2) vCPUid. This vCPUid is allocated by the +L1:: + + H_GUEST_CREATE_VCPU(uint64 flags, + uint64 guestId, + uint64 vcpuId); + Parameters: + Input: + flags: Reserved + guestId: ID obtained from H_GUEST_CREATE + vcpuId: ID of the vCPU to be created. This must be within the + range of 0 to 2047 + Output: + R3: Return code. Notable: + H_Not_Enough_Resources: Unable to create Guest VCPU due to not + enough Hypervisor memory. See H_GUEST_CREATE_GET_STATE(flags = + takeOwnershipOfVcpuState) + +H_GUEST_GET_STATE() +------------------- + +This is called to get state associated with an L2 (Guest-wide or vCPU specific). +This info is passed via the Guest State Buffer (GSB), a standard format as +explained later in this doc, necessary details below: + +This can get either L2 wide or vcpu specific information. Examples of +L2 wide is the timebase offset or process scoped page table +info. Examples of vCPU specific are GPRs or VSRs. A bit in the flags +parameter specifies if this call is L2 wide or vCPU specific and the +IDs in the GSB must match this. + +The L1 provides a pointer to the GSB as a parameter to this call. Also +provided is the L2 and vCPU IDs associated with the state to set. + +The L1 writes only the IDs and sizes in the GSB. L0 writes the +associated values for each ID in the GSB:: + + H_GUEST_GET_STATE(uint64 flags, + uint64 guestId, + uint64 vcpuId, + uint64 dataBuffer, + uint64 dataBufferSizeInBytes); + Parameters: + Input: + flags: + Bit 0: getGuestWideState: Request state of the Guest instead + of an individual VCPU. + Bit 1: takeOwnershipOfVcpuState Indicate the L1 is taking + over ownership of the VCPU state and that the L0 can free + the storage holding the state. The VCPU state will need to + be returned to the Hypervisor via H_GUEST_SET_STATE prior + to H_GUEST_RUN_VCPU being called for this VCPU. The data + returned in the dataBuffer is in a Hypervisor internal + format. + Bits 2-63: Reserved + guestId: ID obtained from H_GUEST_CREATE + vcpuId: ID of the vCPU pass to H_GUEST_CREATE_VCPU + dataBuffer: A L1 real address of the GSB. + If takeOwnershipOfVcpuState, size must be at least the size + returned by ID=0x0001 + dataBufferSizeInBytes: Size of dataBuffer + Output: + R3: Return code + R4: If R3 = H_Invalid_Element_Id: The array index of the bad + element ID. + If R3 = H_Invalid_Element_Size: The array index of the bad + element size. + If R3 = H_Invalid_Element_Value: The array index of the bad + element value. + +H_GUEST_SET_STATE() +------------------- + +This is called to set L2 wide or vCPU specific L2 state. This info is +passed via the Guest State Buffer (GSB), necessary details below: + +This can set either L2 wide or vcpu specific information. Examples of +L2 wide is the timebase offset or process scoped page table +info. Examples of vCPU specific are GPRs or VSRs. A bit in the flags +parameter specifies if this call is L2 wide or vCPU specific and the +IDs in the GSB must match this. + +The L1 provides a pointer to the GSB as a parameter to this call. Also +provided is the L2 and vCPU IDs associated with the state to set. + +The L1 writes all values in the GSB and the L0 only reads the GSB for +this call:: + + H_GUEST_SET_STATE(uint64 flags, + uint64 guestId, + uint64 vcpuId, + uint64 dataBuffer, + uint64 dataBufferSizeInBytes); + Parameters: + Input: + flags: + Bit 0: getGuestWideState: Request state of the Guest instead + of an individual VCPU. + Bit 1: returnOwnershipOfVcpuState Return Guest VCPU state. See + GET_STATE takeOwnershipOfVcpuState + Bits 2-63: Reserved + guestId: ID obtained from H_GUEST_CREATE + vcpuId: ID of the vCPU pass to H_GUEST_CREATE_VCPU + dataBuffer: A L1 real address of the GSB. + If takeOwnershipOfVcpuState, size must be at least the size + returned by ID=0x0001 + dataBufferSizeInBytes: Size of dataBuffer + Output: + R3: Return code + R4: If R3 = H_Invalid_Element_Id: The array index of the bad + element ID. + If R3 = H_Invalid_Element_Size: The array index of the bad + element size. + If R3 = H_Invalid_Element_Value: The array index of the bad + element value. + +H_GUEST_RUN_VCPU() +------------------ + +This is called to run an L2 vCPU. The L2 and vCPU IDs are passed in as +parameters. The vCPU runs with the state set previously using +H_GUEST_SET_STATE(). When the L2 exits, the L1 will resume from this +hcall. + +This hcall also has associated input and output GSBs. Unlike +H_GUEST_{S,G}ET_STATE(), these GSB pointers are not passed in as +parameters to the hcall (This was done in the interest of +performance). The locations of these GSBs must be preregistered using +the H_GUEST_SET_STATE() call with ID 0x0c00 and 0x0c01 (see table +below). + +The input GSB may contain only VCPU specific elements to be set. This +GSB may also contain zero elements (ie 0 in the first 4 bytes of the +GSB) if nothing needs to be set. + +On exit from the hcall, the output buffer is filled with elements +determined by the L0. The reason for the exit is contained in GPR4 (ie +NIP is put in GPR4). The elements returned depend on the exit +type. For example, if the exit reason is the L2 doing a hcall (GPR4 = +0xc00), then GPR3-12 are provided in the output GSB as this is the +state likely needed to service the hcall. If additional state is +needed, H_GUEST_GET_STATE() may be called by the L1. + +To synthesize interrupts in the L2, when calling H_GUEST_RUN_VCPU() +the L1 may set a flag (as a hcall parameter) and the L0 will +synthesize the interrupt in the L2. Alternatively, the L1 may +synthesize the interrupt itself using H_GUEST_SET_STATE() or the +H_GUEST_RUN_VCPU() input GSB to set the state appropriately:: + + H_GUEST_RUN_VCPU(uint64 flags, + uint64 guestId, + uint64 vcpuId, + uint64 dataBuffer, + uint64 dataBufferSizeInBytes); + Parameters: + Input: + flags: + Bit 0: generateExternalInterrupt: Generate an external interrupt + Bit 1: generatePrivilegedDoorbell: Generate a Privileged Doorbell + Bit 2: sendToSystemResetâ: Generate a System Reset Interrupt + Bits 3-63: Reserved + guestId: ID obtained from H_GUEST_CREATE + vcpuId: ID of the vCPU pass to H_GUEST_CREATE_VCPU + Output: + R3: Return code + R4: If R3 = H_Success: The reason L1 VCPU exited (ie. NIA) + 0x000: The VCPU stopped running for an unspecified reason. An + example of this is the Hypervisor stopping a VCPU running + due to an outstanding interrupt for the Host Partition. + 0x980: HDEC + 0xC00: HCALL + 0xE00: HDSI + 0xE20: HISI + 0xE40: HEA + 0xF80: HV Fac Unavail + If R3 = H_Invalid_Element_Id, H_Invalid_Element_Size, or + H_Invalid_Element_Value: R4 is offset of the invalid element + in the input buffer. + +H_GUEST_DELETE() +---------------- + +This is called to delete an L2. All associated vCPUs are also +deleted. No specific vCPU delete call is provided. + +A flag may be provided to delete all guests. This is used to reset the +L0 in the case of kdump/kexec:: + + H_GUEST_DELETE(uint64 flags, + uint64 guestId) + Parameters: + Input: + flags: + Bit 0: deleteAllGuests: deletes all guests + Bits 1-63: Reserved + guestId: ID obtained from H_GUEST_CREATE + Output: + R3: Return code + +Guest State Buffer +================== + +The Guest State Buffer (GSB) is the main method of communicating state +about the L2 between the L1 and L0 via H_GUEST_{G,S}ET() and +H_GUEST_VCPU_RUN() calls. + +State may be associated with a whole L2 (eg timebase offset) or a +specific L2 vCPU (eg. GPR state). Only L2 VCPU state maybe be set by +H_GUEST_VCPU_RUN(). + +All data in the GSB is big endian (as is standard in PAPR) + +The Guest state buffer has a header which gives the number of +elements, followed by the GSB elements themselves. + +GSB header: + ++----------+----------+-------------------------------------------+ +| Offset | Size | Purpose | +| Bytes | Bytes | | ++==========+==========+===========================================+ +| 0 | 4 | Number of elements | ++----------+----------+-------------------------------------------+ +| 4 | | Guest state buffer elements | ++----------+----------+-------------------------------------------+ + +GSB element: + ++----------+----------+-------------------------------------------+ +| Offset | Size | Purpose | +| Bytes | Bytes | | ++==========+==========+===========================================+ +| 0 | 2 | ID | ++----------+----------+-------------------------------------------+ +| 2 | 2 | Size of Value | ++----------+----------+-------------------------------------------+ +| 4 | As above | Value | ++----------+----------+-------------------------------------------+ + +The ID in the GSB element specifies what is to be set. This includes +archtected state like GPRs, VSRs, SPRs, plus also some meta data about +the partition like the timebase offset and partition scoped page +table information. + ++--------+-------+----+--------+----------------------------------+ +| ID | Size | RW | Thread | Details | +| | Bytes | | Guest | | +| | | | Scope | | ++========+=======+====+========+==================================+ +| 0x0000 | | RW | TG | NOP element | ++--------+-------+----+--------+----------------------------------+ +| 0x0001 | 0x08 | R | G | Size of L0 vCPU state. See: | +| | | | | H_GUEST_GET_STATE: | +| | | | | flags = takeOwnershipOfVcpuState | ++--------+-------+----+--------+----------------------------------+ +| 0x0002 | 0x08 | R | G | Size Run vCPU out buffer | ++--------+-------+----+--------+----------------------------------+ +| 0x0003 | 0x04 | RW | G | Logical PVR | ++--------+-------+----+--------+----------------------------------+ +| 0x0004 | 0x08 | RW | G | TB Offset (L1 relative) | ++--------+-------+----+--------+----------------------------------+ +| 0x0005 | 0x18 | RW | G |Partition scoped page tbl info: | +| | | | | | +| | | | |- 0x00 Addr part scope table | +| | | | |- 0x08 Num addr bits | +| | | | |- 0x10 Size root dir | ++--------+-------+----+--------+----------------------------------+ +| 0x0006 | 0x10 | RW | G |Process Table Information: | +| | | | | | +| | | | |- 0x0 Addr proc scope table | +| | | | |- 0x8 Table size. | ++--------+-------+----+--------+----------------------------------+ +| 0x0007-| | | | Reserved | +| 0x0BFF | | | | | ++--------+-------+----+--------+----------------------------------+ +| 0x0C00 | 0x10 | RW | T |Run vCPU Input Buffer: | +| | | | | | +| | | | |- 0x0 Addr of buffer | +| | | | |- 0x8 Buffer Size. | ++--------+-------+----+--------+----------------------------------+ +| 0x0C01 | 0x10 | RW | T |Run vCPU Output Buffer: | +| | | | | | +| | | | |- 0x0 Addr of buffer | +| | | | |- 0x8 Buffer Size. | ++--------+-------+----+--------+----------------------------------+ +| 0x0C02 | 0x08 | RW | T | vCPU VPA Address | ++--------+-------+----+--------+----------------------------------+ +| 0x0C03-| | | | Reserved | +| 0x0FFF | | | | | ++--------+-------+----+--------+----------------------------------+ +| 0x1000-| 0x08 | RW | T | GPR 0-31 | +| 0x101F | | | | | ++--------+-------+----+--------+----------------------------------+ +| 0x1020 | 0x08 | T | T | HDEC expiry TB | ++--------+-------+----+--------+----------------------------------+ +| 0x1021 | 0x08 | RW | T | NIA | ++--------+-------+----+--------+----------------------------------+ +| 0x1022 | 0x08 | RW | T | MSR | ++--------+-------+----+--------+----------------------------------+ +| 0x1023 | 0x08 | RW | T | LR | ++--------+-------+----+--------+----------------------------------+ +| 0x1024 | 0x08 | RW | T | XER | ++--------+-------+----+--------+----------------------------------+ +| 0x1025 | 0x08 | RW | T | CTR | ++--------+-------+----+--------+----------------------------------+ +| 0x1026 | 0x08 | RW | T | CFAR | ++--------+-------+----+--------+----------------------------------+ +| 0x1027 | 0x08 | RW | T | SRR0 | ++--------+-------+----+--------+----------------------------------+ +| 0x1028 | 0x08 | RW | T | SRR1 | ++--------+-------+----+--------+----------------------------------+ +| 0x1029 | 0x08 | RW | T | DAR | ++--------+-------+----+--------+----------------------------------+ +| 0x102A | 0x08 | RW | T | DEC expiry TB | ++--------+-------+----+--------+----------------------------------+ +| 0x102B | 0x08 | RW | T | VTB | ++--------+-------+----+--------+----------------------------------+ +| 0x102C | 0x08 | RW | T | LPCR | ++--------+-------+----+--------+----------------------------------+ +| 0x102D | 0x08 | RW | T | HFSCR | ++--------+-------+----+--------+----------------------------------+ +| 0x102E | 0x08 | RW | T | FSCR | ++--------+-------+----+--------+----------------------------------+ +| 0x102F | 0x08 | RW | T | FPSCR | ++--------+-------+----+--------+----------------------------------+ +| 0x1030 | 0x08 | RW | T | DAWR0 | ++--------+-------+----+--------+----------------------------------+ +| 0x1031 | 0x08 | RW | T | DAWR1 | ++--------+-------+----+--------+----------------------------------+ +| 0x1032 | 0x08 | RW | T | CIABR | ++--------+-------+----+--------+----------------------------------+ +| 0x1033 | 0x08 | RW | T | PURR | ++--------+-------+----+--------+----------------------------------+ +| 0x1034 | 0x08 | RW | T | SPURR | ++--------+-------+----+--------+----------------------------------+ +| 0x1035 | 0x08 | RW | T | IC | ++--------+-------+----+--------+----------------------------------+ +| 0x1036-| 0x08 | RW | T | SPRG 0-3 | +| 0x1039 | | | | | ++--------+-------+----+--------+----------------------------------+ +| 0x103A | 0x08 | W | T | PPR | ++--------+-------+----+--------+----------------------------------+ +| 0x103B | 0x08 | RW | T | MMCR 0-3 | +| 0x103E | | | | | ++--------+-------+----+--------+----------------------------------+ +| 0x103F | 0x08 | RW | T | MMCRA | ++--------+-------+----+--------+----------------------------------+ +| 0x1040 | 0x08 | RW | T | SIER | ++--------+-------+----+--------+----------------------------------+ +| 0x1041 | 0x08 | RW | T | SIER 2 | ++--------+-------+----+--------+----------------------------------+ +| 0x1042 | 0x08 | RW | T | SIER 3 | ++--------+-------+----+--------+----------------------------------+ +| 0x1043 | 0x08 | RW | T | BESCR | ++--------+-------+----+--------+----------------------------------+ +| 0x1044 | 0x08 | RW | T | EBBHR | ++--------+-------+----+--------+----------------------------------+ +| 0x1045 | 0x08 | RW | T | EBBRR | ++--------+-------+----+--------+----------------------------------+ +| 0x1046 | 0x08 | RW | T | AMR | ++--------+-------+----+--------+----------------------------------+ +| 0x1047 | 0x08 | RW | T | IAMR | ++--------+-------+----+--------+----------------------------------+ +| 0x1048 | 0x08 | RW | T | AMOR | ++--------+-------+----+--------+----------------------------------+ +| 0x1049 | 0x08 | RW | T | UAMOR | ++--------+-------+----+--------+----------------------------------+ +| 0x104A | 0x08 | RW | T | SDAR | ++--------+-------+----+--------+----------------------------------+ +| 0x104B | 0x08 | RW | T | SIAR | ++--------+-------+----+--------+----------------------------------+ +| 0x104C | 0x08 | RW | T | DSCR | ++--------+-------+----+--------+----------------------------------+ +| 0x104D | 0x08 | RW | T | TAR | ++--------+-------+----+--------+----------------------------------+ +| 0x104E | 0x08 | RW | T | DEXCR | ++--------+-------+----+--------+----------------------------------+ +| 0x104F | 0x08 | RW | T | HDEXCR | ++--------+-------+----+--------+----------------------------------+ +| 0x1050 | 0x08 | RW | T | HASHKEYR | ++--------+-------+----+--------+----------------------------------+ +| 0x1051 | 0x08 | RW | T | HASHPKEYR | ++--------+-------+----+--------+----------------------------------+ +| 0x1052 | 0x08 | RW | T | CTRL | ++--------+-------+----+--------+----------------------------------+ +| 0x1053-| | | | Reserved | +| 0x1FFF | | | | | ++--------+-------+----+--------+----------------------------------+ +| 0x2000 | 0x04 | RW | T | CR | ++--------+-------+----+--------+----------------------------------+ +| 0x2001 | 0x04 | RW | T | PIDR | ++--------+-------+----+--------+----------------------------------+ +| 0x2002 | 0x04 | RW | T | DSISR | ++--------+-------+----+--------+----------------------------------+ +| 0x2003 | 0x04 | RW | T | VSCR | ++--------+-------+----+--------+----------------------------------+ +| 0x2004 | 0x04 | RW | T | VRSAVE | ++--------+-------+----+--------+----------------------------------+ +| 0x2005 | 0x04 | RW | T | DAWRX0 | ++--------+-------+----+--------+----------------------------------+ +| 0x2006 | 0x04 | RW | T | DAWRX1 | ++--------+-------+----+--------+----------------------------------+ +| 0x2007-| 0x04 | RW | T | PMC 1-6 | +| 0x200c | | | | | ++--------+-------+----+--------+----------------------------------+ +| 0x200D | 0x04 | RW | T | WORT | ++--------+-------+----+--------+----------------------------------+ +| 0x200E | 0x04 | RW | T | PSPB | ++--------+-------+----+--------+----------------------------------+ +| 0x200F-| | | | Reserved | +| 0x2FFF | | | | | ++--------+-------+----+--------+----------------------------------+ +| 0x3000-| 0x10 | RW | T | VSR 0-63 | +| 0x303F | | | | | ++--------+-------+----+--------+----------------------------------+ +| 0x3040-| | | | Reserved | +| 0xEFFF | | | | | ++--------+-------+----+--------+----------------------------------+ +| 0xF000 | 0x08 | R | T | HDAR | ++--------+-------+----+--------+----------------------------------+ +| 0xF001 | 0x04 | R | T | HDSISR | ++--------+-------+----+--------+----------------------------------+ +| 0xF002 | 0x04 | R | T | HEIR | ++--------+-------+----+--------+----------------------------------+ +| 0xF003 | 0x08 | R | T | ASDR | ++--------+-------+----+--------+----------------------------------+ + + +Miscellaneous info +================== + +State not in ptregs/hvregs +-------------------------- + +In the v1 API, some state is not in the ptregs/hvstate. This includes +the vector register and some SPRs. For the L1 to set this state for +the L2, the L1 loads up these hardware registers before the +h_enter_nested() call and the L0 ensures they end up as the L2 state +(by not touching them). + +The v2 API removes this and explicitly sets this state via the GSB. + +L1 Implementation details: Caching state +---------------------------------------- + +In the v1 API, all state is sent from the L1 to the L0 and vice versa +on every h_enter_nested() hcall. If the L0 is not currently running +any L2s, the L0 has no state information about them. The only +exception to this is the location of the partition table, registered +via h_set_partition_table(). + +The v2 API changes this so that the L0 retains the L2 state even when +it's vCPUs are no longer running. This means that the L1 only needs to +communicate with the L0 about L2 state when it needs to modify the L2 +state, or when it's value is out of date. This provides an opportunity +for performance optimisation. + +When a vCPU exits from a H_GUEST_RUN_VCPU() call, the L1 internally +marks all L2 state as invalid. This means that if the L1 wants to know +the L2 state (say via a kvm_get_one_reg() call), it needs call +H_GUEST_GET_STATE() to get that state. Once it's read, it's marked as +valid in L1 until the L2 is run again. + +Also, when an L1 modifies L2 vcpu state, it doesn't need to write it +to the L0 until that L2 vcpu runs again. Hence when the L1 updates +state (say via a kvm_set_one_reg() call), it writes to an internal L1 +copy and only flushes this copy to the L0 when the L2 runs again via +the H_GUEST_VCPU_RUN() input buffer. + +This lazy updating of state by the L1 avoids unnecessary +H_GUEST_{G|S}ET_STATE() calls. |