On Mon, Nov 13, 2017 at 09:32:09AM +0100, Paolo Bonzini wrote: > 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 We don't have a kvm-unit-tests reproducer for this yet, right? I'm considering trying to write one, but I don't want to duplicate work. -- Eduardo
