Subject: Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a
 guest's xcr0
To: David Matlack <dmatlack@google.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, kvm@vger.kernel.org
References: <1457729240-3846-1-git-send-email-dmatlack@google.com>
Cc: pbonzini@redhat.com, mingo@redhat.com, luto@kernel.org, hpa@zytor.com,
        digitaleric@google.com
From: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Message-ID: <56E66C57.9080802@linux.intel.com>
Date: Mon, 14 Mar 2016 15:46:31 +0800
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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? Under this case, kvm_load_guest_fpu()
is not called for every single VM-enter, that means kernel will use guest's xcr0 to
save/restore XSAVE area.

Maybe a simpler fix is just calling __kernel_fpu_begin() when the CPU switches
to vCPU and reverts it when the vCPU is scheduled out or returns to userspace.