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

[Xiao Guangrong]

On 03/16/2016 03:01 AM, David Matlack wrote:
> On Mon, Mar 14, 2016 at 12:46 AM, Xiao Guangrong
> <guangrong.xiao@xxxxxxxxxxxxxxx> 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.ãThat can not stop handler using fpu.