linux-kernel - [PATCH 1/2] KVM: x86: Add helper to retrieve cached value of user return MSR

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Message-Id: <05a018a6997407080b3b7921ba692aa69a720f07.1758166596.git.houwenlong.hwl@antgroup.com>
Date: Thu, 18 Sep 2025 11:38:50 +0800
From: Hou Wenlong <houwenlong.hwl@...group.com>
To: kvm@...r.kernel.org
Cc: Lai Jiangshan <jiangshan.ljs@...group.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] KVM: x86: Add helper to retrieve cached value of user return MSR

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.

Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.com>
---
 arch/x86/include/asm/kvm_host.h | 1 +
 arch/x86/kvm/x86.c              | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index cb86f3cca3e9..2cbb0f446a9b 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_cache(unsigned int index);
 
 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 6d85fbafc679..88d26c86c3b2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -675,6 +675,14 @@ 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_cache(unsigned int slot)
+{
+	struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
+
+	return msrs->values[slot].curr;
+}
+EXPORT_SYMBOL_GPL(kvm_get_user_return_msr_cache);
+
 static void drop_user_return_notifiers(void)
 {
 	struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);

base-commit: 603c090664d350b7fdaffbe8e6a6e43829938458
-- 
2.31.1