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Date:   Fri, 10 Nov 2017 22:42:15 +0100
From:   Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org, yfu@redhat.com,
        Eduardo Habkost <ehabkost@redhat.com>, stable@vger.kernel.org
Subject: Re: [PATCH] KVM: x86: inject exceptions produced by x86_decode_insn
Message-ID: <20171110214214.GI2189@flask>
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2017-11-10 10:49+0100, Paolo Bonzini:
> 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.
> 
> Thanks to Eduardo Habkost for helping in the debugging of this issue.
> 
> Reported-by: Yanan Fu <yfu@redhat.com>
> Cc: Eduardo Habkost <ehabkost@redhat.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

Applied, thanks.

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