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Date:   Wed, 29 Nov 2017 12:44:42 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Eduardo Habkost <ehabkost@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, yfu@...hat.com
Subject: Re: [PATCH] KVM: x86: inject exceptions produced by x86_decode_insn

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@...hat.com>:
>>>> 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.

Paolo

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