KVM: SVM: Use wrmsr for switching between guest and host IA32_XSS on AMD

When the guest can execute the XSAVES/XRSTORS instructions, set the
hardware IA32_XSS MSR to guest/host values on VM-entry/VM-exit.

Note that vcpu->arch.ia32_xss is currently guaranteed to be 0 on AMD,
since there is no way to change it.
Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Change-Id: Id51a782462086e6d7a3ab621838e200f1c005afd
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm.c

@@ -116,6 +116,8 @@ MODULE_DEVICE_TABLE(x86cpu, svm_cpu_id);
 
 static bool erratum_383_found __read_mostly;
 
+static u64 __read_mostly host_xss;
+
 static const u32 host_save_user_msrs[] = {
 #ifdef CONFIG_X86_64
 	MSR_STAR, MSR_LSTAR, MSR_CSTAR, MSR_SYSCALL_MASK, MSR_KERNEL_GS_BASE,
@@ -1409,6 +1411,9 @@ static __init int svm_hardware_setup(void)
 			pr_info("Virtual GIF supported\n");
 	}
 
+	if (boot_cpu_has(X86_FEATURE_XSAVES))
+		rdmsrl(MSR_IA32_XSS, host_xss);
+
 	return 0;
 
 err:
@@ -5598,6 +5603,22 @@ static void svm_cancel_injection(struct kvm_vcpu *vcpu)
 	svm_complete_interrupts(svm);
 }
 
+static void svm_load_guest_xss(struct kvm_vcpu *vcpu)
+{
+	if (kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE) &&
+	    vcpu->arch.xsaves_enabled &&
+	    vcpu->arch.ia32_xss != host_xss)
+		wrmsrl(MSR_IA32_XSS, vcpu->arch.ia32_xss);
+}
+
+static void svm_load_host_xss(struct kvm_vcpu *vcpu)
+{
+	if (kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE) &&
+	    vcpu->arch.xsaves_enabled &&
+	    vcpu->arch.ia32_xss != host_xss)
+		wrmsrl(MSR_IA32_XSS, host_xss);
+}
+
 static void svm_vcpu_run(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
@@ -5637,6 +5658,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
 
 	clgi();
 	kvm_load_guest_xcr0(vcpu);
+	svm_load_guest_xss(vcpu);
 
 	if (lapic_in_kernel(vcpu) &&
 		vcpu->arch.apic->lapic_timer.timer_advance_ns)
@@ -5786,6 +5808,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
 	if (unlikely(svm->vmcb->control.exit_code == SVM_EXIT_NMI))
 		kvm_before_interrupt(&svm->vcpu);
 
+	svm_load_host_xss(vcpu);
 	kvm_put_guest_xcr0(vcpu);
 	stgi();