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

[Xiao Guangrong <guangrong.xiao@linux.intel.com>]

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.