[RFC PATCH v2 06/34] x86/msr: Use the alternatives mechanism to read PMC

Xin Li (Intel)
2025-04-22T08:23:47Z

To eliminate the indirect call overhead introduced by the pv_ops API,
use the alternatives mechanism to read PMC:

1) When built with !CONFIG_XEN_PV, X86_FEATURE_XENPV becomes a
disabled feature, preventing the Xen PMC read code from being
built and ensuring the native code is executed unconditionally.

2) When built with CONFIG_XEN_PV:

2.1) If not running on the Xen hypervisor (!X86_FEATURE_XENPV),
the kernel runtime binary is patched to unconditionally
jump to the native PMC read code.

2.2) If running on the Xen hypervisor (X86_FEATURE_XENPV), the
kernel runtime binary is patched to unconditionally jump
to the Xen PMC read code.

Consequently, remove the pv_ops PMC read API.

Signed-off-by: Xin Li (Intel) <xin@zytor.com>
---
arch/x86/include/asm/msr.h | 31 ++++++++++++++++++++-------
arch/x86/include/asm/paravirt.h | 5 -----
arch/x86/include/asm/paravirt_types.h | 2 --
arch/x86/kernel/paravirt.c | 1 -
arch/x86/xen/enlighten_pv.c | 2 --
drivers/net/vmxnet3/vmxnet3_drv.c | 2 +-
6 files changed, 24 insertions(+), 19 deletions(-)

diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 01dc8e61ef97..33cf506e2fd6 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -8,6 +8,7 @@

#include <asm/asm.h>
#include <asm/errno.h>
+#include <asm/cpufeature.h>
#include <asm/cpumask.h>
#include <uapi/asm/msr.h>
#include <asm/shared/msr.h>
@@ -73,6 +74,10 @@ static inline void do_trace_read_msr(u32 msr, u64 val, int failed) {}
static inline void do_trace_rdpmc(u32 msr, u64 val, int failed) {}
#endif

+#ifdef CONFIG_XEN_PV
+extern u64 xen_read_pmc(int counter);
+#endif
+
/*
* __rdmsr() and __wrmsr() are the two primitives which are the bare minimum MSR
* accessors and should not have any tracing or other functionality piggybacking
@@ -170,16 +175,32 @@ native_write_msr_safe(u32 msr, u32 low, u32 high)
extern int rdmsr_safe_regs(u32 regs[8]);
extern int wrmsr_safe_regs(u32 regs[8]);

-static inline u64 native_read_pmc(int counter)
+static __always_inline u64 native_rdpmcq(int counter)
{
DECLARE_ARGS(val, low, high);

- asm volatile("rdpmc" : EAX_EDX_RET(val, low, high) : "c" (counter));
+ asm_inline volatile("rdpmc" : EAX_EDX_RET(val, low, high) : "c" (counter));
+
if (tracepoint_enabled(rdpmc))
do_trace_rdpmc(counter, EAX_EDX_VAL(val, low, high), 0);
+
return EAX_EDX_VAL(val, low, high);
}

+static __always_inline u64 rdpmcq(int counter)
+{
+#ifdef CONFIG_XEN_PV
+ if (cpu_feature_enabled(X86_FEATURE_XENPV))
+ return xen_read_pmc(counter);
+#endif
+
+ /*
+ * 1) When built with !CONFIG_XEN_PV.
+ * 2) When built with CONFIG_XEN_PV but not running on Xen hypervisor.
+ */
+ return native_rdpmcq(counter);
+}
+
#ifdef CONFIG_PARAVIRT_XXL
#include <asm/paravirt.h>
#else
@@ -233,12 +254,6 @@ static inline int rdmsrq_safe(u32 msr, u64 *p)
*p = native_read_msr_safe(msr, &err);
return err;
}
-
-static __always_inline u64 rdpmcq(int counter)
-{
- return native_read_pmc(counter);
-}
-
#endif /* !CONFIG_PARAVIRT_XXL */

/* Instruction opcode for WRMSRNS supported in binutils >= 2.40 */
diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 590824916394..c7689f5f70d6 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -239,11 +239,6 @@ static inline int rdmsrq_safe(unsigned msr, u64 *p)
return err;
}

-static __always_inline u64 rdpmcq(int counter)
-{
- return PVOP_CALL1(u64, cpu.read_pmc, counter);
-}
-
static inline void paravirt_alloc_ldt(struct desc_struct *ldt, unsigned entries)
{
PVOP_VCALL2(cpu.alloc_ldt, ldt, entries);
diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
index 631c306ce1ff..475f508531d6 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -101,8 +101,6 @@ struct pv_cpu_ops {
u64 (*read_msr_safe)(unsigned int msr, int *err);
int (*write_msr_safe)(unsigned int msr, unsigned low, unsigned high);

- u64 (*read_pmc)(int counter);
-
void (*start_context_switch)(struct task_struct *prev);
void (*end_context_switch)(struct task_struct *next);
#endif
diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 1ccd05d8999f..28d195ad7514 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -132,7 +132,6 @@ struct paravirt_patch_template pv_ops = {
.cpu.write_msr = native_write_msr,
.cpu.read_msr_safe = native_read_msr_safe,
.cpu.write_msr_safe = native_write_msr_safe,
- .cpu.read_pmc = native_read_pmc,
.cpu.load_tr_desc = native_load_tr_desc,
.cpu.set_ldt = native_set_ldt,
.cpu.load_gdt = native_load_gdt,
diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 846b5737d320..9fbe187aff00 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -1236,8 +1236,6 @@ static const typeof(pv_ops) xen_cpu_ops __initconst = {
.read_msr_safe = xen_read_msr_safe,
.write_msr_safe = xen_write_msr_safe,

- .read_pmc = xen_read_pmc,
-
.load_tr_desc = paravirt_nop,
.set_ldt = xen_set_ldt,
.load_gdt = xen_load_gdt,
diff --git a/drivers/net/vmxnet3/vmxnet3_drv.c b/drivers/net/vmxnet3/vmxnet3_drv.c
index 7edd0b5e0e77..8af3b4d7ef4d 100644
--- a/drivers/net/vmxnet3/vmxnet3_drv.c
+++ b/drivers/net/vmxnet3/vmxnet3_drv.c
@@ -151,7 +151,7 @@ static u64
vmxnet3_get_cycles(int pmc)
{
#ifdef CONFIG_X86
- return native_read_pmc(pmc);
+ return native_rdpmcq(pmc);
#else
return 0;
#endif
--
2.49.0