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

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Message-ID: <CALCETrXTQi9Zdv-vtASDHEwo3JFRjqpE04Nug0vQCvZrjkW5nw@mail.gmail.com>
Date:	Tue, 15 Mar 2016 11:27:16 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	David Matlack <dmatlack@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, kvm list <kvm@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Lutomirski <luto@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Eric Northup <digitaleric@...gle.com>
Subject: Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0
 is loaded

On Mon, Mar 14, 2016 at 6:17 AM, Paolo Bonzini <pbonzini@...hat.com> 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