On 05/02/2010 08:38 PM, Brian Gerst wrote:
On Sun, May 2, 2010 at 10:53 AM, Avi Kivity<avi@xxxxxxxxxx> 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.
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