[PATCH v4 17/18] x86/paravirt: Use alternatives for MSR access with paravirt
From: Juergen Gross
Date: Mon Jun 29 2026 - 03:17:28 EST
When not running as Xen PV guest, patch in the optimal MSR instructions
via alternative and use direct calls otherwise.
This will especially have positive effects for performance when not
running as a Xen PV guest with paravirtualization enabled, as there
will be no call overhead for MSR access functions any longer.
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
V3:
- new patch
V4:
- fix build error with clang (kernel test robot)
---
arch/x86/include/asm/paravirt-msr.h | 136 ++++++++++++++++++++------
arch/x86/include/asm/paravirt_types.h | 1 +
2 files changed, 109 insertions(+), 28 deletions(-)
diff --git a/arch/x86/include/asm/paravirt-msr.h b/arch/x86/include/asm/paravirt-msr.h
index 4b71a1cd780c..ba3ee64446db 100644
--- a/arch/x86/include/asm/paravirt-msr.h
+++ b/arch/x86/include/asm/paravirt-msr.h
@@ -27,67 +27,147 @@ extern struct pv_msr_ops pv_ops_msr;
#define PV_CALLEE_SAVE_REGS_MSR_THUNK(func) \
__PV_CALLEE_SAVE_REGS_THUNK(func, ".text", MSR)
+#define ASM_CLRERR "xor %[err],%[err]\n"
+
+#define PV_RDMSR_VAR(__msr, __val, __type, __func, __err) \
+ asm volatile( \
+ "1:\n" \
+ ALTERNATIVE_2(PARAVIRT_CALL, \
+ RDMSR_AND_SAVE_RESULT ASM_CLRERR, X86_FEATURE_ALWAYS, \
+ ALT_CALL_INSTR, ALT_XEN_CALL) \
+ "2:\n" \
+ _ASM_EXTABLE_TYPE_REG(1b, 2b, __type, %[err]) \
+ : [err] "=d" (__err), [val] "=a" (__val), \
+ ASM_CALL_CONSTRAINT \
+ : paravirt_ptr(pv_ops_msr, __func), "c" (__msr) \
+ : "cc")
+
+#define PV_RDMSR_CONST(__msr, __val, __type, __func, __err) \
+ asm volatile( \
+ "1:\n" \
+ ALTERNATIVE_3(PARAVIRT_CALL, \
+ RDMSR_AND_SAVE_RESULT ASM_CLRERR, X86_FEATURE_ALWAYS, \
+ ASM_RDMSR_IMM ASM_CLRERR, X86_FEATURE_MSR_IMM, \
+ ALT_CALL_INSTR, ALT_XEN_CALL) \
+ "2:\n" \
+ _ASM_EXTABLE_TYPE_REG(1b, 2b, __type, %[err]) \
+ : [err] "=d" (__err), [val] "=a" (__val), \
+ ASM_CALL_CONSTRAINT \
+ : paravirt_ptr(pv_ops_msr, __func), \
+ "c" (__msr), [msr] "i" (__msr) \
+ : "cc")
+
+#define PV_WRMSR(__msr, __val, __type, __func, __err) \
+({ \
+ unsigned long rdx = rdx; \
+ asm volatile( \
+ "1:\n" \
+ ALTERNATIVE_2(PARAVIRT_CALL, \
+ "wrmsr;" ASM_CLRERR, X86_FEATURE_ALWAYS, \
+ ALT_CALL_INSTR, ALT_XEN_CALL) \
+ "2:\n" \
+ _ASM_EXTABLE_TYPE_REG(1b, 2b, __type, %[err]) \
+ : [err] "=a" (__err), "=d" (rdx), ASM_CALL_CONSTRAINT \
+ : paravirt_ptr(pv_ops_msr, __func), \
+ "0" (__val), "1" ((__val) >> 32), "c" (__msr) \
+ : "memory", "cc"); \
+})
+
+#define PV_WRMSRNS_VAR(__msr, __val, __type, __func, __err) \
+({ \
+ unsigned long rdx = rdx; \
+ asm volatile( \
+ "1:\n" \
+ ALTERNATIVE_3(PARAVIRT_CALL, \
+ "wrmsr;" ASM_CLRERR, X86_FEATURE_ALWAYS, \
+ ASM_WRMSRNS ASM_CLRERR, X86_FEATURE_WRMSRNS, \
+ ALT_CALL_INSTR, ALT_XEN_CALL) \
+ "2:\n" \
+ _ASM_EXTABLE_TYPE_REG(1b, 2b, __type, %[err]) \
+ : [err] "=a" (__err), "=d" (rdx), ASM_CALL_CONSTRAINT \
+ : paravirt_ptr(pv_ops_msr, __func), \
+ "0" (__val), "1" ((__val) >> 32), "c" (__msr) \
+ : "memory", "cc"); \
+})
+
+#define PV_WRMSRNS_CONST(__msr, __val, __type, __func, __err) \
+({ \
+ unsigned long rdx = rdx; \
+ asm volatile( \
+ "1:\n" \
+ ALTERNATIVE_4(PARAVIRT_CALL, \
+ "wrmsr;" ASM_CLRERR, X86_FEATURE_ALWAYS, \
+ ASM_WRMSRNS ASM_CLRERR, X86_FEATURE_WRMSRNS, \
+ ASM_WRMSRNS_IMM ASM_CLRERR, X86_FEATURE_MSR_IMM,\
+ ALT_CALL_INSTR, ALT_XEN_CALL) \
+ "2:\n" \
+ _ASM_EXTABLE_TYPE_REG(1b, 2b, __type, %[err]) \
+ : [err] "=a" (__err), "=d" (rdx), ASM_CALL_CONSTRAINT \
+ : paravirt_ptr(pv_ops_msr, __func), \
+ [val] "0" (__val), "1" ((__val) >> 32), \
+ "c" (__msr), [msr] "i" (__msr) \
+ : "memory", "cc"); \
+})
+
static __always_inline u64 read_msr(u32 msr)
{
u64 val;
+ u64 err;
- asm volatile(PARAVIRT_CALL
- : "=a" (val), ASM_CALL_CONSTRAINT
- : paravirt_ptr(pv_ops_msr, read_msr), "c" (msr)
- : "rdx");
+ if (__builtin_constant_p(msr))
+ PV_RDMSR_CONST(msr, val, EX_TYPE_RDMSR, read_msr, err);
+ else
+ PV_RDMSR_VAR(msr, val, EX_TYPE_RDMSR, read_msr, err);
return val;
}
static __always_inline void write_msr(u32 msr, u64 val)
{
- asm volatile(PARAVIRT_CALL
- : ASM_CALL_CONSTRAINT
- : paravirt_ptr(pv_ops_msr, write_msr), "c" (msr), "a" (val)
- : "memory", "rdx");
+ u64 err;
+
+ PV_WRMSR(msr, val, EX_TYPE_WRMSR, write_msr, err);
}
static __always_inline void write_msrns(u32 msr, u64 val)
{
- asm volatile(PARAVIRT_CALL
- : ASM_CALL_CONSTRAINT
- : paravirt_ptr(pv_ops_msr, write_msr), "c" (msr), "a" (val)
- : "memory", "rdx");
+ u64 err;
+
+ if (__builtin_constant_p(msr))
+ PV_WRMSRNS_CONST(msr, val, EX_TYPE_WRMSR, write_msr, err);
+ else
+ PV_WRMSRNS_VAR(msr, val, EX_TYPE_WRMSR, write_msr, err);
}
static __always_inline int read_msr_safe(u32 msr, u64 *val)
{
- int err;
+ u64 err;
- asm volatile(PARAVIRT_CALL
- : [err] "=d" (err), "=a" (*val), ASM_CALL_CONSTRAINT
- : paravirt_ptr(pv_ops_msr, read_msr_safe), "c" (msr));
+ if (__builtin_constant_p(msr))
+ PV_RDMSR_CONST(msr, *val, EX_TYPE_RDMSR_SAFE, read_msr_safe, err);
+ else
+ PV_RDMSR_VAR(msr, *val, EX_TYPE_RDMSR_SAFE, read_msr_safe, err);
return err ? -EIO : 0;
}
static __always_inline int write_msr_safe(u32 msr, u64 val)
{
- int err;
+ u64 err;
- asm volatile(PARAVIRT_CALL
- : [err] "=a" (err), ASM_CALL_CONSTRAINT
- : paravirt_ptr(pv_ops_msr, write_msr_safe),
- "c" (msr), "a" (val)
- : "memory", "rdx");
+ PV_WRMSR(msr, val, EX_TYPE_WRMSR_SAFE, write_msr_safe, err);
return err ? -EIO : 0;
}
static __always_inline int write_msrns_safe(u32 msr, u64 val)
{
- int err;
+ u64 err;
- asm volatile(PARAVIRT_CALL
- : [err] "=a" (err), ASM_CALL_CONSTRAINT
- : paravirt_ptr(pv_ops_msr, write_msr_safe),
- "c" (msr), "a" (val)
- : "memory", "rdx");
+ if (__builtin_constant_p(msr))
+ PV_WRMSRNS_CONST(msr, val, EX_TYPE_WRMSR_SAFE, write_msr_safe, err);
+ else
+ PV_WRMSRNS_VAR(msr, val, EX_TYPE_WRMSR_SAFE, write_msr_safe, err);
return err ? -EIO : 0;
}
diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
index 740ea819bbab..54f7c3d8fadf 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -442,6 +442,7 @@ extern struct paravirt_patch_template pv_ops;
#endif /* __ASSEMBLER__ */
#define ALT_NOT_XEN ALT_NOT(X86_FEATURE_XENPV)
+#define ALT_XEN_CALL ALT_DIRECT_CALL(X86_FEATURE_XENPV)
#ifdef CONFIG_X86_32
/* save and restore all caller-save registers, except return value */
--
2.54.0