linux-kernel - [PATCH 05/10] KVM/x86: Add KVM_MSR_RET

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Message-ID: <20251205074537.17072-6-jgross@suse.com>
Date: Fri,  5 Dec 2025 08:45:32 +0100
From: Juergen Gross <jgross@...e.com>
To: linux-kernel@...r.kernel.org,
	x86@...nel.org,
	kvm@...r.kernel.org
Cc: Juergen Gross <jgross@...e.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>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH 05/10] KVM/x86: Add KVM_MSR_RET_* defines for values 0 and 1

For MSR emulation return values only 2 special cases have defines,
while the most used values 0 and 1 don't.

Reason seems to be the maze of function calls of MSR emulation
intertwined with the KVM guest exit handlers, which are using the
values 0 and 1 for other purposes. This even led to the comment above
the already existing defines, warning to use the values 0 and 1 (and
negative errno values) in the MSR emulation at all.

Fact is that MSR emulation and exit handlers are in fact rather well
distinct, with only very few exceptions which are handled in a sane
way.

So add defines for 0 and 1 values of MSR emulation and at the same
time comments where exit handlers are calling into MSR emulation.

The new defines will be used later.

No change of functionality intended.

Signed-off-by: Juergen Gross <jgross@...e.com>
---
 arch/x86/kvm/x86.c |  2 ++
 arch/x86/kvm/x86.h | 10 ++++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e733cb923312..e87963a47aa5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2130,6 +2130,7 @@ static int __kvm_emulate_rdmsr(struct kvm_vcpu *vcpu, u32 msr, int reg,
 	u64 data;
 	int r;
 
+	/* Call MSR emulation. */
 	r = kvm_emulate_msr_read(vcpu, msr, &data);
 
 	if (!r) {
@@ -2171,6 +2172,7 @@ static int __kvm_emulate_wrmsr(struct kvm_vcpu *vcpu, u32 msr, u64 data)
 {
 	int r;
 
+	/* Call MSR emulation. */
 	r = kvm_emulate_msr_write(vcpu, msr, data);
 	if (!r) {
 		trace_kvm_msr_write(msr, data);
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index f3dc77f006f9..e44b6373b106 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -639,15 +639,21 @@ enum kvm_msr_access {
 /*
  * Internal error codes that are used to indicate that MSR emulation encountered
  * an error that should result in #GP in the guest, unless userspace handles it.
- * Note, '1', '0', and negative numbers are off limits, as they are used by KVM
- * as part of KVM's lightly documented internal KVM_RUN return codes.
+ * Note, negative errno values are possible for return values, too.
+ * In case MSR emulation is called from an exit handler, any return value other
+ * than KVM_MSR_RET_OK will normally result in a GP in the guest.
  *
+ * OK		- Emulation succeeded. Must be 0, as in some cases return values
+ *		  of functions returning 0 or -errno will just be passed on.
+ * ERR		- Some error occurred.
  * UNSUPPORTED	- The MSR isn't supported, either because it is completely
  *		  unknown to KVM, or because the MSR should not exist according
  *		  to the vCPU model.
  *
  * FILTERED	- Access to the MSR is denied by a userspace MSR filter.
  */
+#define  KVM_MSR_RET_OK			0
+#define  KVM_MSR_RET_ERR		1
 #define  KVM_MSR_RET_UNSUPPORTED	2
 #define  KVM_MSR_RET_FILTERED		3
 
-- 
2.51.0