Re: [PATCH 2/9] x86/fpu: Hard-disable lazy fpu mode

From: Paolo Bonzini
Date: Wed Oct 05 2016 - 12:10:00 EST

On 05/10/2016 17:59, Andy Lutomirski wrote:
> I actually benchmarked the underlying instructions quite a bit on
> Intel. (Not on AMD, but I doubt the results are very different.)
> Writes to CR0.TS are *incredibly* slow, as are device-not-available
> exceptions. Keep in mind that, while there's a (slow) CLTS
> instruction, there is no corresponding STTS instruction, so we're left
> with a fully serializing, slowly microcoded move to CR0. On SVM, I
> think it's worse, because IIRC SVM doesn't have fancy execution
> controls that let MOV to CR0 avoid exiting.

SVM lets you choose whether to trap on TS and MP; update_cr0_intercept
is where KVM does that (the "selective CR0 write" intercept is always
on, while the "CR0 write" intercept is toggled in that function).

> We're talking a couple
> hundred cycles best case for a TS set/clear pair, and thousands of
> cycles if we actually take a fault.
> In contrast, an unconditional XSAVE + XRSTOR was considerably faster.

Did you also do a comparison against FXSAVE/FXRSTOR (on either pre- or
post-SandyBridge processors)?

But yeah, it's possible that the lack of STTS screws the whole plan,
despite the fpu.preload optimization in switch_fpu_prepare.


> This leads to the counterintuitive result that, if we switch from task
> A to B and back and task A is heavily using the FPU, then it's faster
> to unconditoinally save and restore the full state both ways than it
> is to set and clear TS so we can avoid it.