From ace9429bb58fd418f0c81d4c2835699bddf6bde6 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Thu, 11 Apr 2024 10:27:49 +0200 Subject: Adding upstream version 6.6.15. Signed-off-by: Daniel Baumann --- Documentation/virt/kvm/x86/errata.rst | 50 +++++++++++++++++++++++++++++++++++ 1 file changed, 50 insertions(+) create mode 100644 Documentation/virt/kvm/x86/errata.rst (limited to 'Documentation/virt/kvm/x86/errata.rst') diff --git a/Documentation/virt/kvm/x86/errata.rst b/Documentation/virt/kvm/x86/errata.rst new file mode 100644 index 0000000000..49a05f2474 --- /dev/null +++ b/Documentation/virt/kvm/x86/errata.rst @@ -0,0 +1,50 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======================================= +Known limitations of CPU virtualization +======================================= + +Whenever perfect emulation of a CPU feature is impossible or too hard, KVM +has to choose between not implementing the feature at all or introducing +behavioral differences between virtual machines and bare metal systems. + +This file documents some of the known limitations that KVM has in +virtualizing CPU features. + +x86 +=== + +``KVM_GET_SUPPORTED_CPUID`` issues +---------------------------------- + +x87 features +~~~~~~~~~~~~ + +Unlike most other CPUID feature bits, CPUID[EAX=7,ECX=0]:EBX[6] +(FDP_EXCPTN_ONLY) and CPUID[EAX=7,ECX=0]:EBX]13] (ZERO_FCS_FDS) are +clear if the features are present and set if the features are not present. + +Clearing these bits in CPUID has no effect on the operation of the guest; +if these bits are set on hardware, the features will not be present on +any virtual machine that runs on that hardware. + +**Workaround:** It is recommended to always set these bits in guest CPUID. +Note however that any software (e.g ``WIN87EM.DLL``) expecting these features +to be present likely predates these CPUID feature bits, and therefore +doesn't know to check for them anyway. + +Nested virtualization features +------------------------------ + +TBD + +x2APIC +------ +When KVM_X2APIC_API_USE_32BIT_IDS is enabled, KVM activates a hack/quirk that +allows sending events to a single vCPU using its x2APIC ID even if the target +vCPU has legacy xAPIC enabled, e.g. to bring up hotplugged vCPUs via INIT-SIPI +on VMs with > 255 vCPUs. A side effect of the quirk is that, if multiple vCPUs +have the same physical APIC ID, KVM will deliver events targeting that APIC ID +only to the vCPU with the lowest vCPU ID. If KVM_X2APIC_API_USE_32BIT_IDS is +not enabled, KVM follows x86 architecture when processing interrupts (all vCPUs +matching the target APIC ID receive the interrupt). -- cgit v1.2.3