linux-kernel - Re: [PATCH v4 3/3] KVM: MMU: retry #PF for softmmu

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Message-ID: <4CFCB983.6020503@cn.fujitsu.com>
Date:	Mon, 06 Dec 2010 18:22:59 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v4 3/3] KVM: MMU: retry #PF for softmmu

Hi Avi,

On 12/06/2010 05:48 PM, Avi Kivity wrote:
> On 12/02/2010 11:46 AM, Xiao Guangrong wrote:
>> Retry #PF for softmmu only when the current vcpu has the same cr3 as
>> the time
>> when #PF occurs
>>
>> Changelog:
>>    Just compare cr3 value since It's harmless to instantiate an spte
>> for an
>>    unused translation from Marcelo's comment
>>
> 
> It's not harmless.  We could be in a different process, so we have to
> re-fetch the gpte.  Or we could have switched from one nested guest to
> another.
> 

But it does this in this patch: re-walk guest page table and fixes guest's
shadow page (not use any info when apf is occurred)

>>           if (!eperm&&  !rsvd_fault&&  !(pte&  PT_ACCESSED_MASK)) {
>> +            /*
>> +             * Don't set gpte accessed bit if it's on
>> +             * speculative path.
>> +             */
>> +            if (prefault)
>> +                goto error;
> 
> It's actually legal to set the accessed bit on speculative access.
> 

Oh, you are right, thanks for your reminder.

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