diff options
Diffstat (limited to 'arch/x86/xen/xen-asm_32.S')
| -rw-r--r-- | arch/x86/xen/xen-asm_32.S | 207 |
1 files changed, 207 insertions, 0 deletions
diff --git a/arch/x86/xen/xen-asm_32.S b/arch/x86/xen/xen-asm_32.S new file mode 100644 index 000000000..c15db060a --- /dev/null +++ b/arch/x86/xen/xen-asm_32.S @@ -0,0 +1,207 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +/* + * Asm versions of Xen pv-ops, suitable for direct use. + * + * We only bother with direct forms (ie, vcpu in pda) of the + * operations here; the indirect forms are better handled in C. + */ + +#include <asm/thread_info.h> +#include <asm/processor-flags.h> +#include <asm/segment.h> +#include <asm/asm.h> + +#include <xen/interface/xen.h> + +#include <linux/linkage.h> + +/* Pseudo-flag used for virtual NMI, which we don't implement yet */ +#define XEN_EFLAGS_NMI 0x80000000 + +/* + * This is run where a normal iret would be run, with the same stack setup: + * 8: eflags + * 4: cs + * esp-> 0: eip + * + * This attempts to make sure that any pending events are dealt with + * on return to usermode, but there is a small window in which an + * event can happen just before entering usermode. If the nested + * interrupt ends up setting one of the TIF_WORK_MASK pending work + * flags, they will not be tested again before returning to + * usermode. This means that a process can end up with pending work, + * which will be unprocessed until the process enters and leaves the + * kernel again, which could be an unbounded amount of time. This + * means that a pending signal or reschedule event could be + * indefinitely delayed. + * + * The fix is to notice a nested interrupt in the critical window, and + * if one occurs, then fold the nested interrupt into the current + * interrupt stack frame, and re-process it iteratively rather than + * recursively. This means that it will exit via the normal path, and + * all pending work will be dealt with appropriately. + * + * Because the nested interrupt handler needs to deal with the current + * stack state in whatever form its in, we keep things simple by only + * using a single register which is pushed/popped on the stack. + */ + +.macro POP_FS +1: + popw %fs +.pushsection .fixup, "ax" +2: movw $0, (%esp) + jmp 1b +.popsection + _ASM_EXTABLE(1b,2b) +.endm + +ENTRY(xen_iret) + /* test eflags for special cases */ + testl $(X86_EFLAGS_VM | XEN_EFLAGS_NMI), 8(%esp) + jnz hyper_iret + + push %eax + ESP_OFFSET=4 # bytes pushed onto stack + + /* Store vcpu_info pointer for easy access */ +#ifdef CONFIG_SMP + pushw %fs + movl $(__KERNEL_PERCPU), %eax + movl %eax, %fs + movl %fs:xen_vcpu, %eax + POP_FS +#else + movl %ss:xen_vcpu, %eax +#endif + + /* check IF state we're restoring */ + testb $X86_EFLAGS_IF>>8, 8+1+ESP_OFFSET(%esp) + + /* + * Maybe enable events. Once this happens we could get a + * recursive event, so the critical region starts immediately + * afterwards. However, if that happens we don't end up + * resuming the code, so we don't have to be worried about + * being preempted to another CPU. + */ + setz %ss:XEN_vcpu_info_mask(%eax) +xen_iret_start_crit: + + /* check for unmasked and pending */ + cmpw $0x0001, %ss:XEN_vcpu_info_pending(%eax) + + /* + * If there's something pending, mask events again so we can + * jump back into xen_hypervisor_callback. Otherwise do not + * touch XEN_vcpu_info_mask. + */ + jne 1f + movb $1, %ss:XEN_vcpu_info_mask(%eax) + +1: popl %eax + + /* + * From this point on the registers are restored and the stack + * updated, so we don't need to worry about it if we're + * preempted + */ +iret_restore_end: + + /* + * Jump to hypervisor_callback after fixing up the stack. + * Events are masked, so jumping out of the critical region is + * OK. + */ + je xen_hypervisor_callback + +1: iret +xen_iret_end_crit: + _ASM_EXTABLE(1b, iret_exc) + +hyper_iret: + /* put this out of line since its very rarely used */ + jmp hypercall_page + __HYPERVISOR_iret * 32 + + .globl xen_iret_start_crit, xen_iret_end_crit + +/* + * This is called by xen_hypervisor_callback in entry.S when it sees + * that the EIP at the time of interrupt was between + * xen_iret_start_crit and xen_iret_end_crit. We're passed the EIP in + * %eax so we can do a more refined determination of what to do. + * + * The stack format at this point is: + * ---------------- + * ss : (ss/esp may be present if we came from usermode) + * esp : + * eflags } outer exception info + * cs } + * eip } + * ---------------- <- edi (copy dest) + * eax : outer eax if it hasn't been restored + * ---------------- + * eflags } nested exception info + * cs } (no ss/esp because we're nested + * eip } from the same ring) + * orig_eax }<- esi (copy src) + * - - - - - - - - + * fs } + * es } + * ds } SAVE_ALL state + * eax } + * : : + * ebx }<- esp + * ---------------- + * + * In order to deliver the nested exception properly, we need to shift + * everything from the return addr up to the error code so it sits + * just under the outer exception info. This means that when we + * handle the exception, we do it in the context of the outer + * exception rather than starting a new one. + * + * The only caveat is that if the outer eax hasn't been restored yet + * (ie, it's still on stack), we need to insert its value into the + * SAVE_ALL state before going on, since it's usermode state which we + * eventually need to restore. + */ +ENTRY(xen_iret_crit_fixup) + /* + * Paranoia: Make sure we're really coming from kernel space. + * One could imagine a case where userspace jumps into the + * critical range address, but just before the CPU delivers a + * GP, it decides to deliver an interrupt instead. Unlikely? + * Definitely. Easy to avoid? Yes. The Intel documents + * explicitly say that the reported EIP for a bad jump is the + * jump instruction itself, not the destination, but some + * virtual environments get this wrong. + */ + movl PT_CS(%esp), %ecx + andl $SEGMENT_RPL_MASK, %ecx + cmpl $USER_RPL, %ecx + je 2f + + lea PT_ORIG_EAX(%esp), %esi + lea PT_EFLAGS(%esp), %edi + + /* + * If eip is before iret_restore_end then stack + * hasn't been restored yet. + */ + cmp $iret_restore_end, %eax + jae 1f + + movl 0+4(%edi), %eax /* copy EAX (just above top of frame) */ + movl %eax, PT_EAX(%esp) + + lea ESP_OFFSET(%edi), %edi /* move dest up over saved regs */ + + /* set up the copy */ +1: std + mov $PT_EIP / 4, %ecx /* saved regs up to orig_eax */ + rep movsl + cld + + lea 4(%edi), %esp /* point esp to new frame */ +2: jmp xen_do_upcall + |