From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0
 is loaded
Date: Wed, 16 Mar 2016 13:01:12 +0100
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Cc: David Matlack <dmatlack@google.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	X86 ML <x86@kernel.org>, kvm list <kvm@vger.kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Andrew Lutomirski <luto@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Eric Northup <digitaleric@google.com>
To: Xiao Guangrong <guangrong.xiao@linux.intel.com>,
	Andy Lutomirski <luto@amacapital.net>
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On 16/03/2016 04:55, Xiao Guangrong wrote:
>>>>>
>>>>> 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.
> 
> Relying on interrupt is not easy as XCR0 can not be automatically
> saved/loaded by VMCS... Once interrupt happens, it will use guest's XCR0 anyway.

Right, that's why an xsetbv while interrupts are disabled is appealing.

Paolo