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

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Date:	Wed, 16 Mar 2016 13:01:12 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	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 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