KVM: x86: Allow to update cached values in kvm_user_return_msrs w/o wrmsr

Several MSRs are constant and only used in userspace(ring 3).  But VMs may
have different values.  KVM uses kvm_set_user_return_msr() to switch to
guest's values and leverages user return notifier to restore them when the
kernel is to return to userspace.  To eliminate unnecessary wrmsr, KVM also
caches the value it wrote to an MSR last time.

TDX module unconditionally resets some of these MSRs to architectural INIT
state on TD exit.  It makes the cached values in kvm_user_return_msrs are
inconsistent with values in hardware.  This inconsistency needs to be
fixed.  Otherwise, it may mislead kvm_on_user_return() to skip restoring
some MSRs to the host's values.  kvm_set_user_return_msr() can help correct
this case, but it is not optimal as it always does a wrmsr.  So, introduce
a variation of kvm_set_user_return_msr() to update cached values and skip
that wrmsr.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20250129095902.16391-9-adrian.hunter@intel.com>
Reviewed-by: Xiayao Li <xiaoyao.li@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: gaochaointel

arch/x86/include/asm/kvm_host.h

@@ -2328,6 +2328,7 @@ int kvm_pv_send_ipi(struct kvm *kvm, unsigned long ipi_bitmap_low,
 int kvm_add_user_return_msr(u32 msr);
 int kvm_find_user_return_msr(u32 msr);
 int kvm_set_user_return_msr(unsigned index, u64 val, u64 mask);
+void kvm_user_return_msr_update_cache(unsigned int index, u64 val);
 
 static inline bool kvm_is_supported_user_return_msr(u32 msr)
 {

arch/x86/kvm/x86.c

@@ -636,6 +636,15 @@ static void kvm_user_return_msr_cpu_online(void)
 	}
 }
 
+static void kvm_user_return_register_notifier(struct kvm_user_return_msrs *msrs)
+{
+	if (!msrs->registered) {
+		msrs->urn.on_user_return = kvm_on_user_return;
+		user_return_notifier_register(&msrs->urn);
+		msrs->registered = true;
+	}
+}
+
 int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
 {
 	struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
@@ -649,15 +658,20 @@ int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask)
 		return 1;
 
 	msrs->values[slot].curr = value;
-	if (!msrs->registered) {
-		msrs->urn.on_user_return = kvm_on_user_return;
-		user_return_notifier_register(&msrs->urn);
-		msrs->registered = true;
-	}
+	kvm_user_return_register_notifier(msrs);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(kvm_set_user_return_msr);
 
+void kvm_user_return_msr_update_cache(unsigned int slot, u64 value)
+{
+	struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
+
+	msrs->values[slot].curr = value;
+	kvm_user_return_register_notifier(msrs);
+}
+EXPORT_SYMBOL_GPL(kvm_user_return_msr_update_cache);
+
 static void drop_user_return_notifiers(void)
 {
 	struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);