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

From: Paolo Bonzini
Date: Tue Mar 15 2016 - 15:33:08 EST

On 15/03/2016 19:27, Andy Lutomirski wrote:
> 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.

I am fairly sure it was added in Sandy Bridge, together with AVX. But
what really matters for eager FPU is not xsave, it's xsaveopt, and I
think AMD has never even produced a microprocessor that supports it.

> 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.

It's still a few cache lines. Benchmarks will tell.