Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a guest's xcr0 | From | Xiao Guangrong <> | Date | Wed, 16 Mar 2016 11:43:53 +0800 |
| |
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.
| ![\](/images/icornerr.gif) |