Message-ID: <558250D4.3050505@linux.intel.com>
Date: Thu, 18 Jun 2015 13:02:12 +0800
From: Xiao Guangrong <guangrong.xiao@linux.intel.com>
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To: Paolo Bonzini <pbonzini@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
CC: rkrcmar@redhat.com, bdas@redhat.com
Subject: Re: [PATCH v2 12/13] KVM: x86: add SMM to the MMU role, support SMRAM
 address space
References: <1432746314-50196-1-git-send-email-pbonzini@redhat.com> <1432746314-50196-13-git-send-email-pbonzini@redhat.com> <55766510.7010108@linux.intel.com> <55812D4F.10506@redhat.com>
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On 06/17/2015 04:18 PM, Paolo Bonzini wrote:
>
>
> On 09/06/2015 06:01, Xiao Guangrong wrote:
>>
>>
>> On 05/28/2015 01:05 AM, Paolo Bonzini wrote:
>>> This is now very simple to do.  The only interesting part is a simple
>>> trick to find the right memslot in gfn_to_rmap, retrieving the address
>>> space from the spte role word.  The same trick is used in the auditing
>>> code.
>>>
>>> The comment on top of union kvm_mmu_page_role has been stale forever,
>>
>> Fortunately, we have documented these fields in mmu.txt, please do it for
>> 'smm' as well. :)
>
> Right, done.
>
>>> +        /*
>>> +         * This is left at the top of the word so that
>>> +         * kvm_memslots_for_spte_role can extract it with a
>>> +         * simple shift.  While there is room, give it a whole
>>> +         * byte so it is also faster to load it from memory.
>>> +         */
>>> +        unsigned smm:8;
>>
>> I suspect if we really need this trick, smm is not the hottest filed in
>> this struct anyway.
>
> Note that after these patches it is used by gfn_to_rmap, and hence for
> example rmap_add.

However, role->level is more hotter than role->smm so that it's also a good
candidate for this kind of trick.

And this is only 32 bits which can be operated in a CPU register by a single
memory load, that is why i was worried if it is really needed.


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