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KVM: x86: Add a WARN on TIF_NEED_FPU_LOAD in kvm_load_guest_fpu()

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WARN once in kvm_load_guest_fpu() if TIF_NEED_FPU_LOAD is observed, as
that would mean that KVM is corrupting userspace's FPU by saving
unknown register state into arch.user_fpu.  Add a comment to explain
why KVM WARNs on TIF_NEED_FPU_LOAD instead of implementing logic
similar to fpu__copy().

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2019-12-09 12:05:17 -08:00 committed by Paolo Bonzini
parent f958bd2314
commit 95145c25a7
1 changed files with 7 additions and 0 deletions
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7
arch/x86/kvm/x86.c
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@ -8476,6 +8476,13 @@ static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
{
fpregs_lock();
/*
* Reloading userspace's FPU is handled by kvm_arch_vcpu_load(), both
* for direct calls from userspace (via vcpu_load()) and if this task
* is preempted (via kvm_sched_in()) between vcpu_load() and now.
*/
WARN_ON_ONCE(test_thread_flag(TIF_NEED_FPU_LOAD));
copy_fpregs_to_fpstate(vcpu->arch.user_fpu);
/* PKRU is separately restored in kvm_x86_ops->run. */
__copy_kernel_to_fpregs(&vcpu->arch.guest_fpu->state,