2017-11-30 2:42 GMT+08:00 Eduardo Habkost <ehabkost@xxxxxxxxxx>: > On Wed, Nov 29, 2017 at 12:44:42PM +0100, Paolo Bonzini wrote: >> On 29/11/2017 12:44, Eduardo Habkost wrote: >> > 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. >> >> No, I haven't written one yet. > > The reproducer (not a full test case) is quite simple, see patch below. I can also have a look if there is a formal test case. :) Regards, Wanpeng Li
