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. 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 >
