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SubjectRe: [PATCH v4 0/10] x86/xsaves: Fix XSAVES known issues
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On 04/30/2016 12:53 AM, Ingo Molnar wrote:
> We can still use the compacted area handling instructions, because presumably
> those are the fastest and are also the most optimized ones? But I wouldn't use
> them to do dynamic allocation: just allocate the maximum possible FPU save area at
> task creation time and never again worry about that detail.
>
> Ok?

Sounds sane to me.

BTW, I hacked up your "fpu performance" to compare XSAVE vs. XSAVES:

> [ 0.048347] x86/fpu: Cost of: XSAVE insn : 127 cycles
> [ 0.049134] x86/fpu: Cost of: XSAVES insn : 113 cycles
> [ 0.048492] x86/fpu: Cost of: XRSTOR insn : 120 cycles
> [ 0.049267] x86/fpu: Cost of: XRSTORS insn : 102 cycles

So I guess we can add that to the list of things that XSAVES is good
for. Granted, the real-world benefit is probably hard to measure
because the cache residency of the XSAVE buffer isn't as good when
_actually_ context switching, but this at least shows a small
theoretical advantage for XSAVES.

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