On 13/11/2017 08:15, Wanpeng Li wrote: > 2017-11-10 17:49 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>: >> Sometimes, a processor might execute an instruction while another >> processor is updating the page tables for that instruction's code page, >> but before the TLB shootdown completes. The interesting case happens >> if the page is in the TLB. >> >> In general, the processor will succeed in executing the instruction and >> nothing bad happens. However, what if the instruction is an MMIO access? >> If *that* happens, KVM invokes the emulator, and the emulator gets the >> updated page tables. If the update side had marked the code page as non >> present, the page table walk then will fail and so will x86_decode_insn. >> >> Unfortunately, even though kvm_fetch_guest_virt is correctly returning >> X86EMUL_PROPAGATE_FAULT, x86_decode_insn's caller treats the failure as >> a fatal error if the instruction cannot simply be reexecuted (as is the >> case for MMIO). And this in fact happened sometimes when rebooting >> Windows 2012r2 guests. Just checking ctxt->have_exception and injecting >> the exception if true is enough to fix the case. > > I found the only place which can set ctxt->have_exception is in the > function x86_emulate_insn(), and x86_decode_insn() will not set > ctxt->have_exception even if kvm_fetch_guest_virt() returns > X86_EMUL_PROPAGATE_FAULT. Hmm, you're right. Looks like Yanan has been (un)lucky when trying out this patch! :( Yanan, can you double check that you can reproduce the issue with an unpatched kernel? I will work on a kvm-unit-tests testcsae Paolo > Regards, > Wanpeng Li > >> >> Thanks to Eduardo Habkost for helping in the debugging of this issue. >> >> Reported-by: Yanan Fu <yfu@xxxxxxxxxx> >> Cc: Eduardo Habkost <ehabkost@xxxxxxxxxx> >> Cc: stable@xxxxxxxxxxxxxxx >> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> >> --- >> arch/x86/kvm/x86.c | 2 ++ >> 1 file changed, 2 insertions(+) >> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> index 34c85aa2e2d1..6dbed9022797 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c >> @@ -5722,6 +5722,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, >> if (reexecute_instruction(vcpu, cr2, write_fault_to_spt, >> emulation_type)) >> return EMULATE_DONE; >> + if (ctxt->have_exception && inject_emulated_exception(vcpu)) >> + return EMULATE_DONE; >> if (emulation_type & EMULTYPE_SKIP) >> return EMULATE_FAIL; >> return handle_emulation_failure(vcpu); >> -- >> 1.8.3.1 >>
