[PATCH v2 1/2] KVM: x86: Add helper to retrieve current value of user return MSR

[Sean Christopherson <seanjc@google.com>]

From: Hou Wenlong <houwenlong.hwl@antgroup.com>

In the user return MSR support, the cached value is always the hardware
value of the specific MSR. Therefore, add a helper to retrieve the
cached value, which can replace the need for RDMSR, for example, to
allow SEV-ES guests to restore the correct host hardware value without
using RDMSR.

Cc: stable@vger.kernel.org
Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
[sean: drop "cache" from the name, make it a one-liner, tag for stable]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/kvm_host.h | 1 +
 arch/x86/kvm/x86.c              | 6 ++++++
 2 files changed, 7 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 17772513b9cc..14236006266b 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2376,6 +2376,7 @@ 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);
+u64 kvm_get_user_return_msr(unsigned int slot);
 
 static inline bool kvm_is_supported_user_return_msr(u32 msr)
 {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e07936efacd4..801bf6172a21 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -675,6 +675,12 @@ void kvm_user_return_msr_update_cache(unsigned int slot, u64 value)
 }
 EXPORT_SYMBOL_GPL(kvm_user_return_msr_update_cache);
 
+u64 kvm_get_user_return_msr(unsigned int slot)
+{
+	return this_cpu_ptr(user_return_msrs)->values[slot].curr;
+}
+EXPORT_SYMBOL_GPL(kvm_get_user_return_msr);
+
 static void drop_user_return_notifiers(void)
 {
 	struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
-- 
2.51.0.470.ga7dc726c21-goog