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

From: Paolo Bonzini
Date: Wed Mar 16 2016 - 08:01:26 EST

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.