KVM: SVM: restore host save area from assembly

Allow access to the percpu area via the GS segment base, which is
needed in order to access the saved host spec_ctrl value.  In linux-next
FILL_RETURN_BUFFER also needs to access percpu data.

For simplicity, the physical address of the save area is added to struct
svm_cpu_data.

Cc: stable@vger.kernel.org
Fixes: a149180f ("x86: Add magic AMD return-thunk")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Analyzed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/kvm-asm-offsets.c

@@ -18,6 +18,7 @@ static void __used common(void)
 		OFFSET(SVM_current_vmcb, vcpu_svm, current_vmcb);
 		OFFSET(SVM_vmcb01, vcpu_svm, vmcb01);
 		OFFSET(KVM_VMCB_pa, kvm_vmcb_info, pa);
+		OFFSET(SD_save_area_pa, svm_cpu_data, save_area_pa);
 	}
 
 	if (IS_ENABLED(CONFIG_KVM_INTEL)) {

arch/x86/kvm/svm/svm.c

@@ -592,7 +592,7 @@ static int svm_hardware_enable(void)
 
 	wrmsrl(MSR_EFER, efer | EFER_SVME);
 
-	wrmsrl(MSR_VM_HSAVE_PA, __sme_page_pa(sd->save_area));
+	wrmsrl(MSR_VM_HSAVE_PA, sd->save_area_pa);
 
 	if (static_cpu_has(X86_FEATURE_TSCRATEMSR)) {
 		/*
@@ -648,6 +648,7 @@ static void svm_cpu_uninit(int cpu)
 
 	kfree(sd->sev_vmcbs);
 	__free_page(sd->save_area);
+	sd->save_area_pa = 0;
 	sd->save_area = NULL;
 }
 
@@ -665,6 +666,7 @@ static int svm_cpu_init(int cpu)
 	if (ret)
 		goto free_save_area;
 
+	sd->save_area_pa = __sme_page_pa(sd->save_area);
 	return 0;
 
 free_save_area:
@@ -1450,7 +1452,7 @@ static void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
 	 * Save additional host state that will be restored on VMEXIT (sev-es)
 	 * or subsequent vmload of host save area.
 	 */
-	vmsave(__sme_page_pa(sd->save_area));
+	vmsave(sd->save_area_pa);
 	if (sev_es_guest(vcpu->kvm)) {
 		struct sev_es_save_area *hostsa;
 		hostsa = (struct sev_es_save_area *)(page_address(sd->save_area) + 0x400);
@@ -3905,14 +3907,10 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu)
 
 	guest_state_enter_irqoff();
 
-	if (sev_es_guest(vcpu->kvm)) {
+	if (sev_es_guest(vcpu->kvm))
 		__svm_sev_es_vcpu_run(svm);
-	} else {
-		struct svm_cpu_data *sd = per_cpu_ptr(&svm_data, vcpu->cpu);
-
+	else
 		__svm_vcpu_run(svm);
-		vmload(__sme_page_pa(sd->save_area));
-	}
 
 	guest_state_exit_irqoff();
 }

arch/x86/kvm/svm/svm.h

@@ -287,6 +287,8 @@ struct svm_cpu_data {
 	struct kvm_ldttss_desc *tss_desc;
 
 	struct page *save_area;
+	unsigned long save_area_pa;
+
 	struct vmcb *current_vmcb;
 
 	/* index = sev_asid, value = vmcb pointer */

arch/x86/kvm/svm/svm_ops.h

@@ -61,9 +61,4 @@ static __always_inline void vmsave(unsigned long pa)
 	svm_asm1(vmsave, "a" (pa), "memory");
 }
 
-static __always_inline void vmload(unsigned long pa)
-{
-	svm_asm1(vmload, "a" (pa), "memory");
-}
-
 #endif /* __KVM_X86_SVM_OPS_H */

arch/x86/kvm/svm/vmenter.S

@@ -49,6 +49,14 @@ SYM_FUNC_START(__svm_vcpu_run)
 #endif
 	push %_ASM_BX
 
+	/*
+	 * Save variables needed after vmexit on the stack, in inverse
+	 * order compared to when they are needed.
+	 */
+
+	/* Needed to restore access to percpu variables.  */
+	__ASM_SIZE(push) PER_CPU_VAR(svm_data + SD_save_area_pa)
+
 	/* Save @svm. */
 	push %_ASM_ARG1
 
@@ -124,6 +132,11 @@ SYM_FUNC_START(__svm_vcpu_run)
 5:	vmsave %_ASM_AX
 6:
 
+	/* Restores GSBASE among other things, allowing access to percpu data.  */
+	pop %_ASM_AX
+7:	vmload %_ASM_AX
+8:
+
 #ifdef CONFIG_RETPOLINE
 	/* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
 	FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE
@@ -187,10 +200,14 @@ SYM_FUNC_START(__svm_vcpu_run)
 50:	cmpb $0, kvm_rebooting
 	jne 6b
 	ud2
+70:	cmpb $0, kvm_rebooting
+	jne 8b
+	ud2
 
 	_ASM_EXTABLE(1b, 10b)
 	_ASM_EXTABLE(3b, 30b)
 	_ASM_EXTABLE(5b, 50b)
+	_ASM_EXTABLE(7b, 70b)
 
 SYM_FUNC_END(__svm_vcpu_run)