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Subject: Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0
 is loaded
To: Paolo Bonzini <pbonzini@redhat.com>, Andy Lutomirski <luto@amacapital.net>
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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>
From: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Message-ID: <56E8D919.7080002@linux.intel.com>
Date: Wed, 16 Mar 2016 11:55:05 +0800
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On 03/16/2016 03:32 AM, Paolo Bonzini wrote:
>
>
> On 15/03/2016 19:27, Andy Lutomirski wrote:
>> On Mon, Mar 14, 2016 at 6:17 AM, Paolo Bonzini <pbonzini@redhat.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.

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.