On Sat, Feb 1, 2014 at 5:57 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
> Twiddling CR0.TS is pretty slow if we're not taking advantage of it.
Immaterial.
We *already* avoid twiddling TS if it's not needed.
It is true that we used to twiddle it at every context switch (and
then twiddle it *again* if we decided that we'd want to pre-load the
FPU state anyway, and avoid the extra fault).
But that was fixed, and if we switch from a task that had math state
to another task that has math state, we leave TS alone.
But Suresh apparently hits on the real issue:
> not all the state under xsave adhers to cr0.TS/DNA rules
which if so is sad but yes, makes CR0.TS no longer sufficient.
Linus
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