Re: [PATCH] Lazy FPU save/restore for VT

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>   
>> Howdy,
>>
>> Attached patch implements lazy-fpu save/restore for VT.  VMEXIT time 
>> improves by about 10% (~550 cycles).  I had to do a couple more things 
>> to get it working than I had to do with SVM.  I changed the CR0 host 
>> mask to be all 1's so that any attempt to write to CR0 causes a 
>> VMEXIT.  I don't think there are any remaining bits now that we want 
>> to trap TS that are safe for the guest to access and are in the 
>> fast-paths.
>>     
>
> Great!  With a couple more msrs we'll get to 50% off what we had two 
> weeks ago.
>
>   
>> Since we're trapping TS, I had to implement CLTS exit handling.  
>> vcpu->cr0 also had a rather bizarre life cycle.  After a set_cr0, it 
>> was a proper shadow of the guest's CR0.  However, after a decache_cr0, 
>> it would contain the host's version of the bits covered by the CR0 
>> host mask so it was no longer a proper shadow.
>>
>> I got rid of the CR0 caching and made vcpu->cr0 always be equivalent 
>> to CR0_READ_SHADOW.  Once these changes were made, the rest of the 
>> patch was much like the SVM one.
>>
>>  
>> -	if ((intr_info & INTR_INFO_INTR_TYPE_MASK) == 0x200) { /* nmi */
>> +	switch (intr_info & INTR_INFO_VECTOR_MASK) {
>> +	case NMI_VECTOR:
>>  		asm ("int $2");
>>  		return 1;
>> +	case NM_VECTOR:
>> +		vcpu->fpu_active = 1;
>> +		vmcs_clear_bits(EXCEPTION_BITMAP, 1 << NM_VECTOR);
>> +		if (!(vcpu->cr0 & CR0_TS_MASK))
>> +			vmcs_clear_bits(GUEST_CR0, CR0_TS_MASK);
>> +		return 1;
>>  	}
>>   
>>     
>
> I'd like to be conservative here and not depend just on the vector: 
> check type == 2 for nmi, and type == 6, vector == NM for #NM.  In fact, 
> I see that is_page_fault() and is_external_interrupt() implement 
> something like that already.
>   

Okay.

> Also, can you split the patch into the cr0 cache fix and the lazy fpu?
>   

Sure, I was considering that already before I submitted.

Regards,

Anthony Liguori


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