Re: [PATCH] nestedhvm: ASID emulation

[Christoph Egger <Christoph.Egger@amd.com>]

On 04/13/11 17:05, Keir Fraser wrote:
> On 13/04/2011 15:26, "Christoph Egger"<Christoph.Egger@amd.com>  wrote:
>
>> On 04/13/11 15:27, Keir Fraser wrote:
>>> On 13/04/2011 11:37, "Christoph Egger"<Christoph.Egger@amd.com>   wrote:
>>>
>> We talk about a win of about 1000 cycles per VMRUN and another 1000
>> cycles per VMEXIT emulation.
>>
>> That's a speedup of about 10% for each VMRUN and about 20% for each
>> VMEXIT emulation.
>
> Is this measurable on a macro benchmark?

I measured this with xentrace while l2 guest is booting.

The speedup is noticable on the end-user side just by the feeling on how 
fast the l2 guest reacts on user input.


> I mean this looks like a micro-optimisation on a feature that noone is going
> to use for serious performance work anyway.
>
>> 4. nestedhvm is enabled and we are going to run l2 guest
>>
>> We run the l1 guest in the last call. The asid generation may have
>> changed by then. In this case the current nv_n2asid number is stale
>> and the value of data->next_asid is<= of nv->nv_n2asid.
>
> How do you know for sure that next_asid will be<= nv_n2asid in this case?
> What if other VCPUs have run meanwhile, and next_asid has been incremented
> multiple times until it is greather than nv_n2asid?

In that case  curr->arch.hvm_vcpu.asid_generation is not equal to 
data->core_asid_generation  and a new hw ASID number is assigned
regardless of the values of data->next_asid and nv_n2asid.

Christoph

>
>   -- Keir
>
>> The the value of nv->nv_n2asid is valid if l1 guest doesn't change
>> the virtual asid (= asid number in the virtual vmcb) and
>> data->next_asid is larger than nv->nv_n2asid. In this case
>> just reuse the same hw ASID that has been used from the last
>> VMRUN emulation.
>>
>> 5. nestedhvm is enabled and we are going to run l2 guest again
>>
>> The same hw ASID should be reused unless the generation changed because
>> the nestedp2m got flushed or the vcpu moved to a different physical cpu,
>> for example.
>> But the hw ASID number may never match the hw ASID used to run the l1 guest.
>>
>>
>> In all cases we have to verify that the
>>
>>> I wouldn't bother fixing #2 unless there's a convincing answer for #1.
>>>
>>>    -- Keir
>>
>
>
>


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