Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a guest's xcr0

From	Xiao Guangrong <guangrong.xiao@linux.intel.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a guest's xcr0
Date	2016-03-16 04:50 +0100
Message-ID	<rdaD9-3du-7@gated-at.bofh.it> (permalink)
References	<rbCat-3jh-1@gated-at.bofh.it> <rcvqi-KB-21@gated-at.bofh.it> <rd2vT-6kZ-1@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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.

Thread

[PATCH 0/1] KVM: x86: using the fpu in interrupt context with a guest's xcr0 David Matlack <dmatlack@google.com> - 2016-03-11 21:50 +0100
  [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0 is loaded David Matlack <dmatlack@google.com> - 2016-03-11 21:50 +0100
    Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0  is loaded Andy Lutomirski <luto@amacapital.net> - 2016-03-11 22:20 +0100
      Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0  is loaded David Matlack <dmatlack@google.com> - 2016-03-11 22:40 +0100
        Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0  is loaded Paolo Bonzini <pbonzini@redhat.com> - 2016-03-14 14:20 +0100
          Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0  is loaded Andy Lutomirski <luto@amacapital.net> - 2016-03-15 19:30 +0100
            Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0  is loaded Paolo Bonzini <pbonzini@redhat.com> - 2016-03-15 20:40 +0100
              Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0  is loaded Xiao Guangrong <guangrong.xiao@linux.intel.com> - 2016-03-16 05:00 +0100
                Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0  is loaded Paolo Bonzini <pbonzini@redhat.com> - 2016-03-16 13:10 +0100
  Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a  guest's xcr0 Xiao Guangrong <guangrong.xiao@linux.intel.com> - 2016-03-14 08:50 +0100
    Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a  guest's xcr0 David Matlack <dmatlack@google.com> - 2016-03-15 20:10 +0100
      Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a  guest's xcr0 Xiao Guangrong <guangrong.xiao@linux.intel.com> - 2016-03-16 04:50 +0100
        Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a  guest's xcr0 Andy Lutomirski <luto@amacapital.net> - 2016-03-16 04:50 +0100
          Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a  guest's xcr0 David Matlack <dmatlack@google.com> - 2016-03-16 18:20 +0100
        Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a  guest's xcr0 David Matlack <dmatlack@google.com> - 2016-03-16 18:10 +0100

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