Re: [PATCH] KVM: x86: inject exceptions produced by x86_decode_insn

[Eduardo Habkost]

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