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Subject: Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a
 guest's xcr0
To: David Matlack <dmatlack@google.com>
References: <1457729240-3846-1-git-send-email-dmatlack@google.com>
 <56E66C57.9080802@linux.intel.com>
 <CALzav=dKwN8uojO73D+QtOoTNmvHS=X=AoMKX1rCZJUho2k0Yg@mail.gmail.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>
From: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Message-ID: <56E8D679.9020305@linux.intel.com>
Date: Wed, 16 Mar 2016 11:43:53 +0800
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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.