Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0 is loaded

From: Andy Lutomirski
Date: Tue Mar 15 2016 - 14:27:43 EST

On Mon, Mar 14, 2016 at 6:17 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> On 11/03/2016 22:33, David Matlack wrote:
>> > Is this better than just always keeping the host's XCR0 loaded outside
>> > if the KVM interrupts-disabled region?
>> Probably not. AFAICT KVM does not rely on it being loaded outside that
>> region. xsetbv isn't insanely expensive, is it? Maybe to minimize the
>> time spent with interrupts disabled it was put outside.
>> I do like that your solution would be contained to KVM.
> I agree with Andy. We do want a fix for recent kernels because of the
> !eager_fpu case that Guangrong mentioned.
> Paolo
> ps: while Andy is planning to kill lazy FPU, I want to benchmark it with
> KVM... Remember that with a single pre-xsave host in your cluster, your
> virt management might happily default your VMs to a Westmere or Nehalem
> CPU model. GCC might be a pretty good testbench for this (e.g. a kernel
> compile with very high make -j), because outside of the lexer (which
> plays SIMD games) it never uses the FPU.

Aren't pre-xsave CPUs really, really old? A brief search suggests
that Intel Core added it somewhere in the middle of the cycle.

For pre-xsave, it could indeed hurt performance a tiny bit under
workloads that use the FPU and then stop completely because the
xsaveopt and init optimizations aren't available. But even that is
probably a very small effect, especially because pre-xsave CPUs have
smaller FPU state sizes.

Andy Lutomirski
AMA Capital Management, LLC