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

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Date:	Mon, 18 Aug 2014 22:40:02 -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 Mon, Aug 18, 2014 at 10:19 PM, Xiao Guangrong
<xiaoguangrong@...ux.vnet.ibm.com> wrote:
> On 08/19/2014 01:00 PM, David Matlack wrote:
>> On Mon, Aug 18, 2014 at 9:41 PM, Xiao Guangrong
>> <xiaoguangrong@...ux.vnet.ibm.com> wrote:
>>> On 08/19/2014 12:31 PM, David Matlack wrote:
>>>> The single line patch I suggested was only intended to fix the "forever
>>>> incorrectly exit mmio".
>>>
>>> My patch also fixes this case and that does not doubly increase the
>>> number. I think this is the better one.
>>
>> I prefer doubly increasing the generation for this reason: the updated boolean
>> requires extra code on the "client-side" to check if there's an update in
>> progress. And that makes it easy to get wrong. In fact, your patch
>> forgot to check the updated bit in mark_mmio_spte(). Doubly increasing the
>> generation requires no "client-side" code to work.
>
> No, the updated patch is used to fix case 2 which i draw the scenario in
> the last mail. I mean the original patch in this patchset which just
> increase the number after srcu-sync.
>
> Then could you tell me that your approach can do but my original patch can not?

It avoids publishing new memslots with an old generation number attached to
them (even if it only lasts for a short period of time). Do you have a reason
why you don't want to doubly increase the generation?
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