linux-kernel - Re: [PATCH 1/2] KVM: fix cache stale memslot info with correct mmio generation number

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Message-ID: <CALzav=c1OS+TUjr0S_4BmkPicRxgKv2QhfOB+jWPt0VNEmugHQ@mail.gmail.com>
Date:	Tue, 19 Aug 2014 18:03:43 -0700
From:	David Matlack <dmatlack@...gle.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Gleb Natapov <gleb@...nel.org>,
	Avi Kivity <avi.kivity@...il.com>, mtosatti@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: fix cache stale memslot info with correct mmio
 generation number

On Tue, Aug 19, 2014 at 5:29 PM, Xiao Guangrong
<xiaoguangrong@...ux.vnet.ibm.com> wrote:
> On 08/19/2014 05:03 PM, Paolo Bonzini wrote:
>> Il 19/08/2014 10:50, Xiao Guangrong ha scritto:
>>> Okay, what confused me it that it seems that the single line patch
>>> is ok to you. :)
>>
>> No, it was late and I was confused. :)
>>
>>> Now, do we really need to care the case 2? like David said:
>>> "Sorry I didn't explain myself very well: Since we can get a single wrong
>>> mmio exit no matter what, it has to be handled in userspace. So my point
>>> was, it doesn't really help to fix that one very specific way that it can
>>> happen, because it can just happen in other ways. (E.g. update memslots
>>> occurs after is_noslot_pfn() and before mmio exit)."
>>>
>>> What's your idea?
>>>
>>>> I think if you always treat the low bit as zero in mmio sptes, you can
>>>> do that without losing a bit of the generation.
>>>
>>> What's you did is avoiding cache a invalid generation number into spte, but
>>> actually if we can figure it out when we check mmio access, it's ok. Like the
>>> updated patch i posted should fix it, that way avoids doubly increase the number.
>>
>> Yes.
>>
>>> Okay, if you're interested increasing the number doubly, there is the simpler
>>> one:
>>
>> This wastes a bit in the mmio spte though.  My idea is to increase the
>> memslots generation twice, but drop the low bit in the mmio spte.
>
> Yeah, really smart idea. :)
>
> Paolo/David, would you mind making a patch for this (+ the comments in David's
> patch)?

Paolo, since it was your idea would you like to write it? I don't mind either
way.

>
> Please feel free to add my:
> Reviewed-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
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