Re: [PATCH] Lazy FPU save/restore

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

Avi Kivity wrote:
> Avi Kivity wrote:
>>>
>>> I'll have to try and setup a 64 bit system.  I cannot reproduce on 
>>> my 32bit system with your test program.
>>
>> It may be related to 64-bit (as that uses sse for floating point), or 
>> to the fact that I use ssh on a remote host, which causes more 
>> context switches.  I'll try to reproduce on 32 bits with my setup.
>>
>
> It doesn't reproduce on a 32-bit host.  However, it does reproduce on 
> a 32-bit guest on a 64-bit host, and on a 32-bit user load running in 
> a 64-bit guest on a 64-bit host.

Thanks, I'll setup a 64 bit partition and start debugging.  I'm a bit 
confused though, I thought CR0.TS even covers SIMD instructions?

I'll take a look at Xen and see if it's doing something else.

Regards,

Anthony Liguori

> I'm attaching an updated test program which makes it easier to spot 
> the failures.  You only need to run it on the guest, although running 
> it on the host as well increases the failure rate significantly.
>
> ------------------------------------------------------------------------
>
>
> #include <stdio.h>
> #include <math.h>
>
> double test_fpu_once()
> {
>     int i;
>     double f = 0;
>
>     for (i = 0; i < 10000000; ++i)
> 	f += 1 / (1.0 + i);
>     return f;
> }
>
> void test_fpu()
> {
>     double a, b;
>     int runs;
>
>     runs = 0;
>     a = test_fpu_once();
>     while (1) {
> 	b = test_fpu_once();
> 	if (fabs(a - b) > 1e-9)
> 	    printf("error: %20.16f -> %20.16f\n", a, b);
> 	a = b;
> 	if (++runs % 100 == 0)
> 	    printf("runs: %8d\n", runs);
>     }
> }
>
> int main(int ac, char **av)
> {
>     test_fpu();
>     return 0;
> }
>   


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