KVM: x86: fix maintenance of guest/host xcr0 state

[Marcelo Tosatti <mtosatti@redhat.com>]

** Untested **.

Emulation of xcr0 writes zero guest_xcr0_loaded variable so that
subsequent VM-entry reloads CPU's xcr0 with guests xcr0 value.

However, this is incorrect because guest_xcr0_loaded variable is 
read to decide whether to reload hosts xcr0.

In case the vcpu thread is scheduled out after the guest_xcr0_loaded = 0
assignment, and scheduler decides to preload FPU:

switch_to
{
  __switch_to
    __math_state_restore
      restore_fpu_checking
        fpu_restore_checking
          if (use_xsave())
              fpu_xrstor_checking
		xrstor64 with CPU's xcr0 == guests xcr0

Fix by properly restoring hosts xcr0 during emulation of xcr0 writes.

Analyzed-by: Ulrich Obergfell <uobergfe@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 999d124..222926a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -555,6 +555,25 @@ void kvm_lmsw(struct kvm_vcpu *vcpu, unsigned long msw)
 }
 EXPORT_SYMBOL_GPL(kvm_lmsw);
 
+static void kvm_load_guest_xcr0(struct kvm_vcpu *vcpu)
+{
+	if (kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE) &&
+			!vcpu->guest_xcr0_loaded) {
+		/* kvm_set_xcr() also depends on this */
+		xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
+		vcpu->guest_xcr0_loaded = 1;
+	}
+}
+
+static void kvm_put_guest_xcr0(struct kvm_vcpu *vcpu)
+{
+	if (vcpu->guest_xcr0_loaded) {
+		if (vcpu->arch.xcr0 != host_xcr0)
+			xsetbv(XCR_XFEATURE_ENABLED_MASK, host_xcr0);
+		vcpu->guest_xcr0_loaded = 0;
+	}
+}
+
 int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
 {
 	u64 xcr0;
@@ -571,8 +590,8 @@ int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
 		return 1;
 	if (xcr0 & ~host_xcr0)
 		return 1;
+	kvm_put_guest_xcr0(vcpu);
 	vcpu->arch.xcr0 = xcr0;
-	vcpu->guest_xcr0_loaded = 0;
 	return 0;
 }
 
@@ -5600,25 +5619,6 @@ static void inject_pending_event(struct kvm_vcpu *vcpu)
 	}
 }
 
-static void kvm_load_guest_xcr0(struct kvm_vcpu *vcpu)
-{
-	if (kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE) &&
-			!vcpu->guest_xcr0_loaded) {
-		/* kvm_set_xcr() also depends on this */
-		xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
-		vcpu->guest_xcr0_loaded = 1;
-	}
-}
-
-static void kvm_put_guest_xcr0(struct kvm_vcpu *vcpu)
-{
-	if (vcpu->guest_xcr0_loaded) {
-		if (vcpu->arch.xcr0 != host_xcr0)
-			xsetbv(XCR_XFEATURE_ENABLED_MASK, host_xcr0);
-		vcpu->guest_xcr0_loaded = 0;
-	}
-}
-
 static void process_nmi(struct kvm_vcpu *vcpu)
 {
 	unsigned limit = 2;