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From: David Matlack <dmatlack@google.com>
Date: Wed, 16 Mar 2016 10:09:10 -0700
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Subject: Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a
 guest's xcr0
To: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>, kvm list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Eric Northup <digitaleric@google.com>
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On Tue, Mar 15, 2016 at 8:43 PM, Xiao Guangrong
<guangrong.xiao@linux.intel.com> wrote:
>
>
> On 03/16/2016 03:01 AM, David Matlack wrote:
>>
>> On Mon, Mar 14, 2016 at 12:46 AM, Xiao Guangrong
>> <guangrong.xiao@linux.intel.com> wrote:
>>>
>>> On 03/12/2016 04:47 AM, David Matlack wrote:
>>>
>>>> I have not been able to trigger this bug on Linux 4.3, and suspect
>>>> it is due to this commit from Linux 4.2:
>>>>
>>>> 653f52c kvm,x86: load guest FPU context more eagerly
>>>>
>>>> With this commit, as long as the host is using eagerfpu, the guest's
>>>> fpu is always loaded just before the guest's xcr0 (vcpu->fpu_active
>>>> is always 1 in the following snippet):
>>>>
>>>> 6569         if (vcpu->fpu_active)
>>>> 6570                 kvm_load_guest_fpu(vcpu);
>>>> 6571         kvm_load_guest_xcr0(vcpu);
>>>>
>>>> When the guest's fpu is loaded, irq_fpu_usable() returns false.
>>>
>>> Er, i did not see that commit introduced this change.
>>>
>>>>
>>>> We've included our workaround for this bug, which applies to Linux 3.11.
>>>> It does not apply cleanly to HEAD since the fpu subsystem was refactored
>>>> in Linux 4.2. While the latest kernel does not look vulnerable, we may
>>>> want to apply a fix to the vulnerable stable kernels.
>>>
>>> Is the latest kvm safe if we use !eager fpu?
>>
>> Yes I believe so. When !eagerfpu, interrupted_kernel_fpu_idle()
>> returns "!current->thread.fpu.fpregs_active && (read_cr0() &
>> X86_CR0_TS)". This should ensure the interrupt handler never does
>> XSAVE/XRSTOR with the guest's xcr0.
>
> interrupted_kernel_fpu_idle() returns true if KVM-based hypervisor (e.g.
> QEMU)
> is not using fpu.　That can not stop handler using fpu.

You are correct, the interrupt handler can still use the fpu. But
kernel_fpu_{begin,end} will not execute XSAVE / XRSTOR.