On 22.04.25 10:21, Xin Li (Intel) wrote:
To eliminate the indirect call overhead introduced by the pv_ops API, use the alternatives mechanism to read PMC:
Which indirect call overhead? The indirect call is patched via the alternative mechanism to a direct one.
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.
Without CONFIG_XEN_PV CONFIG_PARAVIRT_XXL is not selected, resulting in native code anyway.
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.
I don't see the value of this patch. It adds more #ifdef and code lines without any real gain. In case the x86 maintainers think it is still worth it, I won't object. Juergen
Signed-off-by: Xin Li (Intel) <xin@xxxxxxxxx> --- 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
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