Re: [PATCH] Lazy FPU save/restore

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

Avi Kivity wrote:
> Avi Kivity wrote:
>> Anthony Liguori wrote:
>>> The following patch saves the host FPU state and loads the guests 
>>> FPU state if !(CR0.MP == 1 && CR0.TS == 1).
>>>
>>> When CR0.MP == 1 && CR0.TS == 1, all FPU activity will generate 
>>> exceptions.  OS's use these exceptions to implement lazy FPU loading 
>>> to improve context switch time.  Since any FPU activity will 
>>> generate traps, we don't have to worry about the guest modifying the 
>>> host FPU state.
>>>
>>> My microbenchmark of choice uses FPU operations so I think the 
>>> results are currently tainted.  I've only tested on a 32bit SVM system.
>>>
>>> Signed-off-by: Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>
>>>
>>
>> Applied, thanks.
>>
>
> ... and reverted.
>
> Running an fpu load on both guest and host shows corruption (both on 
> the guest and host).  The load is
>
>
>> #include <stdio.h>
>>
>> void test_fpu()
>> {
>>     int i;
>>     double f = 0;
>>
>>     for (i = 0; i < 10000000; ++i)
>>     f += 1 / (1.0 + i);
>>     printf("%20.16f\n", f);
>> }
>>
>> int main(int ac, char **av)
>> {
>>     while (1)
>>     test_fpu();
>> }
>
> Compiled statically with -O2 on 64-bit.  I don't know what's wrong -- 
> the code looks correct (well, on Intel it wants KVM_GUEST_CR0_MASK to 
> include CR0_MP_MASK and CR0_TS_MASK, but it fails with that too, and 
> it fails on AMD as well).

I'll have to try and setup a 64 bit system.  I cannot reproduce on my 
32bit system with your test program.

Thanks for figuring this out!

Regards,

Anthony Liguori


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