KVM: VMX: Use flag to indicate "active" uret MSRs instead of sorting list

Explicitly flag a uret MSR as needing to be loaded into hardware instead of
resorting the list of "active" MSRs and tracking how many MSRs in total
need to be loaded.  The only benefit to sorting the list is that the loop
to load MSRs during vmx_prepare_switch_to_guest() doesn't need to iterate
over all supported uret MRS, only those that are active.  But that is a
pointless optimization, as the most common case, running a 64-bit guest,
will load the vast majority of MSRs.  Not to mention that a single WRMSR is
far more expensive than iterating over the list.

Providing a stable list order obviates the need to track a given MSR's
"slot" in the per-CPU list of user return MSRs; all lists simply use the
same ordering.  Future patches will take advantage of the stable order to
further simplify the related code.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210504171734.1434054-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -458,8 +458,9 @@ static unsigned long host_idt_base;
  * Though SYSCALL is only supported in 64-bit mode on Intel CPUs, kvm
  * will emulate SYSCALL in legacy mode if the vendor string in guest
  * CPUID.0:{EBX,ECX,EDX} is "AuthenticAMD" or "AMDisbetter!" To
- * support this emulation, IA32_STAR must always be included in
- * vmx_uret_msrs_list[], even in i386 builds.
+ * support this emulation, MSR_STAR is included in the list for i386,
+ * but is never loaded into hardware.  MSR_CSTAR is also never loaded
+ * into hardware and is here purely for emulation purposes.
  */
 static u32 vmx_uret_msrs_list[] = {
 #ifdef CONFIG_X86_64
@@ -702,18 +703,12 @@ static bool is_valid_passthrough_msr(u32 msr)
 	return r;
 }
 
-static inline int __vmx_find_uret_msr(struct vcpu_vmx *vmx, u32 msr)
+static inline int __vmx_find_uret_msr(u32 msr)
 {
 	int i;
 
-	/*
-	 * Note, vmx->guest_uret_msrs is the same size as vmx_uret_msrs_list,
-	 * but is ordered differently.  The MSR is matched against the list of
-	 * supported uret MSRs using "slot", but the index that is returned is
-	 * the index into guest_uret_msrs.
-	 */
 	for (i = 0; i < vmx_nr_uret_msrs; ++i) {
-		if (vmx_uret_msrs_list[vmx->guest_uret_msrs[i].slot] == msr)
+		if (vmx_uret_msrs_list[i] == msr)
 			return i;
 	}
 	return -1;
@@ -723,7 +718,7 @@ struct vmx_uret_msr *vmx_find_uret_msr(struct vcpu_vmx *vmx, u32 msr)
 {
 	int i;
 
-	i = __vmx_find_uret_msr(vmx, msr);
+	i = __vmx_find_uret_msr(msr);
 	if (i >= 0)
 		return &vmx->guest_uret_msrs[i];
 	return NULL;
@@ -732,13 +727,14 @@ struct vmx_uret_msr *vmx_find_uret_msr(struct vcpu_vmx *vmx, u32 msr)
 static int vmx_set_guest_uret_msr(struct vcpu_vmx *vmx,
 				  struct vmx_uret_msr *msr, u64 data)
 {
+	unsigned int slot = msr - vmx->guest_uret_msrs;
 	int ret = 0;
 
 	u64 old_msr_data = msr->data;
 	msr->data = data;
-	if (msr - vmx->guest_uret_msrs < vmx->nr_active_uret_msrs) {
+	if (msr->load_into_hardware) {
 		preempt_disable();
-		ret = kvm_set_user_return_msr(msr->slot, msr->data, msr->mask);
+		ret = kvm_set_user_return_msr(slot, msr->data, msr->mask);
 		preempt_enable();
 		if (ret)
 			msr->data = old_msr_data;
@@ -1090,7 +1086,7 @@ static bool update_transition_efer(struct vcpu_vmx *vmx)
 		return false;
 	}
 
-	i = __vmx_find_uret_msr(vmx, MSR_EFER);
+	i = __vmx_find_uret_msr(MSR_EFER);
 	if (i < 0)
 		return false;
 
@@ -1252,11 +1248,14 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
 	 */
 	if (!vmx->guest_uret_msrs_loaded) {
 		vmx->guest_uret_msrs_loaded = true;
-		for (i = 0; i < vmx->nr_active_uret_msrs; ++i)
-			kvm_set_user_return_msr(vmx->guest_uret_msrs[i].slot,
+		for (i = 0; i < vmx_nr_uret_msrs; ++i) {
+			if (!vmx->guest_uret_msrs[i].load_into_hardware)
+				continue;
+
+			kvm_set_user_return_msr(i,
 						vmx->guest_uret_msrs[i].data,
 						vmx->guest_uret_msrs[i].mask);
-
+		}
 	}
 
     	if (vmx->nested.need_vmcs12_to_shadow_sync)
@@ -1763,19 +1762,16 @@ static void vmx_queue_exception(struct kvm_vcpu *vcpu)
 	vmx_clear_hlt(vcpu);
 }
 
-static void vmx_setup_uret_msr(struct vcpu_vmx *vmx, unsigned int msr)
+static void vmx_setup_uret_msr(struct vcpu_vmx *vmx, unsigned int msr,
+			       bool load_into_hardware)
 {
-	struct vmx_uret_msr tmp;
-	int from, to;
+	struct vmx_uret_msr *uret_msr;
 
-	from = __vmx_find_uret_msr(vmx, msr);
-	if (from < 0)
+	uret_msr = vmx_find_uret_msr(vmx, msr);
+	if (!uret_msr)
 		return;
-	to = vmx->nr_active_uret_msrs++;
 
-	tmp = vmx->guest_uret_msrs[to];
-	vmx->guest_uret_msrs[to] = vmx->guest_uret_msrs[from];
-	vmx->guest_uret_msrs[from] = tmp;
+	uret_msr->load_into_hardware = load_into_hardware;
 }
 
 /*
@@ -1785,30 +1781,36 @@ static void vmx_setup_uret_msr(struct vcpu_vmx *vmx, unsigned int msr)
  */
 static void setup_msrs(struct vcpu_vmx *vmx)
 {
-	vmx->guest_uret_msrs_loaded = false;
-	vmx->nr_active_uret_msrs = 0;
 #ifdef CONFIG_X86_64
+	bool load_syscall_msrs;
+
 	/*
 	 * The SYSCALL MSRs are only needed on long mode guests, and only
 	 * when EFER.SCE is set.
 	 */
-	if (is_long_mode(&vmx->vcpu) && (vmx->vcpu.arch.efer & EFER_SCE)) {
-		vmx_setup_uret_msr(vmx, MSR_STAR);
-		vmx_setup_uret_msr(vmx, MSR_LSTAR);
-		vmx_setup_uret_msr(vmx, MSR_SYSCALL_MASK);
-	}
+	load_syscall_msrs = is_long_mode(&vmx->vcpu) &&
+			    (vmx->vcpu.arch.efer & EFER_SCE);
+
+	vmx_setup_uret_msr(vmx, MSR_STAR, load_syscall_msrs);
+	vmx_setup_uret_msr(vmx, MSR_LSTAR, load_syscall_msrs);
+	vmx_setup_uret_msr(vmx, MSR_SYSCALL_MASK, load_syscall_msrs);
 #endif
-	if (update_transition_efer(vmx))
-		vmx_setup_uret_msr(vmx, MSR_EFER);
+	vmx_setup_uret_msr(vmx, MSR_EFER, update_transition_efer(vmx));
 
-	if (guest_cpuid_has(&vmx->vcpu, X86_FEATURE_RDTSCP)  ||
-	    guest_cpuid_has(&vmx->vcpu, X86_FEATURE_RDPID))
-		vmx_setup_uret_msr(vmx, MSR_TSC_AUX);
+	vmx_setup_uret_msr(vmx, MSR_TSC_AUX,
+			   guest_cpuid_has(&vmx->vcpu, X86_FEATURE_RDTSCP) ||
+			   guest_cpuid_has(&vmx->vcpu, X86_FEATURE_RDPID));
 
-	vmx_setup_uret_msr(vmx, MSR_IA32_TSX_CTRL);
+	vmx_setup_uret_msr(vmx, MSR_IA32_TSX_CTRL, true);
 
 	if (cpu_has_vmx_msr_bitmap())
 		vmx_update_msr_bitmap(&vmx->vcpu);
+
+	/*
+	 * The set of MSRs to load may have changed, reload MSRs before the
+	 * next VM-Enter.
+	 */
+	vmx->guest_uret_msrs_loaded = false;
 }
 
 static u64 vmx_write_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)

arch/x86/kvm/vmx/vmx.h

@@ -36,7 +36,7 @@ struct vmx_msrs {
 };
 
 struct vmx_uret_msr {
-	unsigned int slot; /* The MSR's slot in kvm_user_return_msrs. */
+	bool load_into_hardware;
 	u64 data;
 	u64 mask;
 };