Re: [PATCH 1/2] x86: eliminate TS_XSAVE

Avi Kivity
2010-05-02T17:44:56Z

On 05/02/2010 08:38 PM, Brian Gerst wrote:
> On Sun, May 2, 2010 at 10:53 AM, Avi Kivity<avi@redhat.com> wrote:
>
>> The fpu code currently uses current->thread_info->status& TS_XSAVE as
>> a way to distinguish between XSAVE capable processors and older processors.
>> The decision is not really task specific; instead we use the task status to
>> avoid a global memory reference - the value should be the same across all
>> threads.
>>
>> Eliminate this tie-in into the task structure by using an alternative
>> instruction keyed off the XSAVE cpu feature; this results in shorter and
>> faster code, without introducing a global memory reference.
>>
> I think you should either just use cpu_has_xsave, or extend this use
> of alternatives to all cpu features. It doesn't make sense to only do
> it for xsave.
>

I was trying to avoid a performance regression relative to the current
code, as it appears that some care was taken to avoid the memory reference.

I agree that it's probably negligible compared to the save/restore
code. If the x86 maintainers agree as well, I'll replace it with
cpu_has_xsave.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.