[PATCH 1/4] KVM: x86: Handle TIF_NEED_FPU_LOAD in kvm_{load,put}_guest_fpu()

[Sean Christopherson <sean.j.christopherson@xxxxxxxxx>]

Handle TIF_NEED_FPU_LOAD similar to how fpu__copy() handles the flag
when duplicating FPU state to a new task struct.  TIF_NEED_FPU_LOAD can
be set any time control is transferred out of KVM, be it voluntarily,
e.g. if I/O is triggered during a KVM call to get_user_pages, or
involuntarily, e.g. if softirq runs after an IRQ occurs.  Therefore,
KVM must account for TIF_NEED_FPU_LOAD whenever it is (potentially)
accessing CPU FPU state.

Fixes: 5f409e20b7945 ("x86/fpu: Defer FPU state load until return to userspace")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
 arch/x86/kvm/x86.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cf917139de6b..0c7211491f98 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8476,8 +8476,20 @@ static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
 {
 	fpregs_lock();
 
-	copy_fpregs_to_fpstate(vcpu->arch.user_fpu);
-	/* PKRU is separately restored in kvm_x86_ops->run.  */
+	/*
+	 * If userspace's FPU state is not resident in the CPU registers, just
+	 * memcpy() from current, else save CPU state directly to user_fpu.
+	 */
+	if (test_thread_flag(TIF_NEED_FPU_LOAD))
+		memcpy(&vcpu->arch.user_fpu->state, &current->thread.fpu.state,
+		       fpu_kernel_xstate_size);
+	else
+		copy_fpregs_to_fpstate(vcpu->arch.user_fpu);
+
+	/*
+	 * Load guest's FPU state to the CPU registers.  PKRU is separately
+	 * loaded in kvm_x86_ops->run.
+	 */
 	__copy_kernel_to_fpregs(&vcpu->arch.guest_fpu->state,
 				~XFEATURE_MASK_PKRU);
 
@@ -8492,7 +8504,16 @@ static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
 {
 	fpregs_lock();
 
-	copy_fpregs_to_fpstate(vcpu->arch.guest_fpu);
+	/*
+	 * If guest's FPU state is not resident in the CPU registers, just
+	 * memcpy() from current, else save CPU state directly to guest_fpu.
+	 */
+	if (test_thread_flag(TIF_NEED_FPU_LOAD))
+		memcpy(&vcpu->arch.guest_fpu->state, &current->thread.fpu.state,
+		       fpu_kernel_xstate_size);
+	else
+		copy_fpregs_to_fpstate(vcpu->arch.guest_fpu);
+
 	copy_kernel_to_fpregs(&vcpu->arch.user_fpu->state);
 
 	fpregs_mark_activate();
-- 
2.24.1