Messages in this thread | | | Subject | Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a guest's xcr0 | From | Xiao Guangrong <> | 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.
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