Re: [PATCH v2 12/13] KVM: x86: add SMM to the MMU role, support SMRAM address space

From	Xiao Guangrong <guangrong.xiao@linux.intel.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2 12/13] KVM: x86: add SMM to the MMU role, support SMRAM address space
Date	2015-06-18 07:10 +0200
Message-ID	<pCAfn-71J-1@gated-at.bofh.it> (permalink)
References	<puN05-1yD-3@gated-at.bofh.it> <puN06-1yD-21@gated-at.bofh.it> <pzj1n-3vn-5@gated-at.bofh.it> <pCgJI-44s-9@gated-at.bofh.it>
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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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Thread

Re: [PATCH v2 12/13] KVM: x86: add SMM to the MMU role, support SMRAM  address space Xiao Guangrong <guangrong.xiao@linux.intel.com> - 2015-06-09 06:10 +0200
  Re: [PATCH v2 12/13] KVM: x86: add SMM to the MMU role, support SMRAM  address space Xiao Guangrong <guangrong.xiao@linux.intel.com> - 2015-06-18 07:10 +0200
    Re: [PATCH v2 12/13] KVM: x86: add SMM to the MMU role, support SMRAM  address space Paolo Bonzini <pbonzini@redhat.com> - 2015-06-18 09:10 +0200

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