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: <56E6BA06.7000907@redhat.com>
Date:	Mon, 14 Mar 2016 14:17:58 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	David Matlack <dmatlack@...gle.com>,
	Andy Lutomirski <luto@...capital.net>
Cc:	"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 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.