Re: [PATCH] Lazy FPU save/restore

Avi Kivity
2007-02-18T14:25:17Z

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).

--
error compiling committee.c: too many arguments to function