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: <56E86366.10403@redhat.com>
Date:	Tue, 15 Mar 2016 20:32:54 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
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 15/03/2016 19:27, Andy Lutomirski wrote:
> 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.

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.

Paolo