To add trace points to msr accesses we need to include linux/tracepoint.h. Unfortunately this causes hellish include loops when with the msr inlines in asm/msr.h, which are included all over. I tried to fix several of them, but eventually gave up.

This patch moves the MSR functions out of line. A MSR access is typically 40-100 cycles or even slower, a call is a few cycles at best, so the additional function call is not really significant.

Kernel text size is neutral:

11852945 1671656 1822720 15347321 ea2e79 vmlinux-no-msr
11852969 1671656 1822720 15347345 ea2e91 vmlinux-msr

As requested, some benchmarking on the difference to inline MSR (including the trace points from the next patch):

The absolute differences are fairly low, 6-8 cycles for out of line + trace point. 6-7% on Haswell. On Avoton the percentages are higher because the base costs are lower, but the absolute cycle deltas are very low too and in the same range.

I think it's reasonable to spend 6-8 cycles/call for much better debuggability. In fact looking at the traces already exposed a number of optimization possibilities for optimizing away unnecessary accesses, that should give much higher gains.

haswell:
136 cycles ool wrmsr
128 cycles inline wrmsr 6%
90 cycles ool rdmsr
84 cycles inline rdmsr 7%

avoton:

68 cycles ool wrmsr
54 cycles inline wrmsr 20%
60 cycles ool rdmsr
44 cycles inline rdmsr 26%

Signed-off-by Andi Kleen <[email protected]>

---
 arch/x86/include/asm/msr.h | 51 ++++----------------------------------------
 arch/x86/lib/msr.c         | 53 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+), 47 deletions(-)

arch/x86/include/asm/msr.h +4 ~~-47~~

index de36f22..99d6864 100644
@@ -57,53 +57,10 @@ static inline unsigned long long native_read_tscp(unsigned int *aux)
 #define EAX_EDX_RET(val, low, high)	"=A" (val)
 #endif
 
-static inline unsigned long long native_read_msr(unsigned int msr)
-{
-	DECLARE_ARGS(val, low, high);
-
-	asm volatile("rdmsr" : EAX_EDX_RET(val, low, high) : "c" (msr));
-	return EAX_EDX_VAL(val, low, high);
-}
-
-static inline unsigned long long native_read_msr_safe(unsigned int msr,
-						      int *err)
-{
-	DECLARE_ARGS(val, low, high);
-
-	asm volatile("2: rdmsr ; xor %[err],%[err]\n"
-		     "1:\n\t"
-		     ".section .fixup,\"ax\"\n\t"
-		     "3:  mov %[fault],%[err] ; jmp 1b\n\t"
-		     ".previous\n\t"
-		     _ASM_EXTABLE(2b, 3b)
-		     : [err] "=r" (*err), EAX_EDX_RET(val, low, high)
-		     : "c" (msr), [fault] "i" (-EIO));
-	return EAX_EDX_VAL(val, low, high);
-}
-
-static inline void native_write_msr(unsigned int msr,
-				    unsigned low, unsigned high)
-{
-	asm volatile("wrmsr" : : "c" (msr), "a"(low), "d" (high) : "memory");
-}
-
-/* Can be uninlined because referenced by paravirt */
-notrace static inline int native_write_msr_safe(unsigned int msr,
-					unsigned low, unsigned high)
-{
-	int err;
-	asm volatile("2: wrmsr ; xor %[err],%[err]\n"
-		     "1:\n\t"
-		     ".section .fixup,\"ax\"\n\t"
-		     "3:  mov %[fault],%[err] ; jmp 1b\n\t"
-		     ".previous\n\t"
-		     _ASM_EXTABLE(2b, 3b)
-		     : [err] "=a" (err)
-		     : "c" (msr), "0" (low), "d" (high),
-		       [fault] "i" (-EIO)
-		     : "memory");
-	return err;
-}
+extern unsigned long long native_read_msr(unsigned int msr);
+extern unsigned long long native_read_msr_safe(unsigned int msr, int *err);
+extern int native_write_msr_safe(unsigned int msr, unsigned low, unsigned high);
+extern void native_write_msr(unsigned int msr, unsigned low, unsigned high);
 
 extern unsigned long long native_read_tsc(void);

arch/x86/lib/msr.c +53 -0

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Signature · 1 line

1.9.3

Andi Kleen

author

2015-03-20 00:29 UTC #2 lore raw

PATCH 2/3 x86: Add trace point for MSR accesses

In reply to Andi Kleen

From: Andi Kleen <[email protected]>

For debugging low level code interacting with the CPU it is often useful to trace the MSR read/writes. This gives a concise summary of PMU and other operations.

perf has an ad-hoc way to do this using trace_printk, but it's somewhat limited (and also now spews ugly messages when enabled)

Instead define real trace points for all MSR accesses.

This adds two new trace point: read_msr and write_msr. They also report if the access faulted (if *_safe is used)

This allows filtering and triggering on specific
MSR values, which allows various more advanced
debugging techniques.

All the values are well defined in the CPU documentation.

I only added it to native MSR accesses in C, not paravirtualized or in entry*.S (which is not too interesting)

Signed-off-by Andi Kleen <[email protected]>

---
 arch/x86/lib/msr.c         | 14 ++++++++++++--
 include/trace/events/msr.h | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+), 2 deletions(-)
 create mode 100644 include/trace/events/msr.h

arch/x86/lib/msr.c +12 -2

index 7eed044..29d1952 100644
@@ -1,6 +1,8 @@
 #include <linux/module.h>
 #include <linux/preempt.h>
 #include <asm/msr.h>
+#define CREATE_TRACE_POINTS
+#include <trace/events/msr.h>
 
 struct msr *msrs_alloc(void)
 {

@@ -111,16 +113,20 @@ int msr_clear_bit(u32 msr, u8 bit)
 
 inline unsigned long long native_read_msr(unsigned int msr)
 {
+	unsigned long long lval;
 	DECLARE_ARGS(val, low, high);
 
 	asm volatile("rdmsr" : EAX_EDX_RET(val, low, high) : "c" (msr));
-	return EAX_EDX_VAL(val, low, high);
+	lval = EAX_EDX_VAL(val, low, high);
+	trace_read_msr(msr, lval, 0);
+	return lval;
 }
 EXPORT_SYMBOL(native_read_msr);
 
 inline unsigned long long native_read_msr_safe(unsigned int msr,
 						      int *err)
 {
+	unsigned long long lval;
 	DECLARE_ARGS(val, low, high);
 
 	asm volatile("2: rdmsr ; xor %[err],%[err]\n"

@@ -131,7 +137,9 @@ inline unsigned long long native_read_msr_safe(unsigned int msr,
 		     _ASM_EXTABLE(2b, 3b)
 		     : [err] "=r" (*err), EAX_EDX_RET(val, low, high)
 		     : "c" (msr), [fault] "i" (-EIO));
-	return EAX_EDX_VAL(val, low, high);
+	lval = EAX_EDX_VAL(val, low, high);
+	trace_read_msr(msr, lval, *err);
+	return lval;
 }
 EXPORT_SYMBOL(native_read_msr_safe);

@@ -139,6 +147,7 @@ inline void native_write_msr(unsigned int msr,
 				    unsigned low, unsigned high)
 {
 	asm volatile("wrmsr" : : "c" (msr), "a"(low), "d" (high) : "memory");
+	trace_write_msr(msr, ((u64)high << 32 | low), 0);
 }
 EXPORT_SYMBOL(native_write_msr);

@@ -158,6 +167,7 @@ notrace inline int native_write_msr_safe(unsigned int msr,
 		     : "c" (msr), "0" (low), "d" (high),
 		       [fault] "i" (-EIO)
 		     : "memory");
+	trace_write_msr(msr, ((u64)high << 32 | low), err);
 	return err;
 }
 EXPORT_SYMBOL(native_write_msr_safe);

include/trace/events/msr.h new file +46 -0

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Signature · 1 line

1.9.3

Andi Kleen

author

2015-03-20 00:29 UTC #3 lore raw

PATCH 3/3 x86, perf: Trace rdpmc too

In reply to Andi Kleen

From: Andi Kleen <[email protected]>

perf uses RDPMC to read the performance counters, so it's useful to trace that too. Add a trace point for RDPMC too, similar to the existing MSR ones. Since there is only a single call of rdpmc in the whole kernel (in perf)
just add the trace statement to that call, instead of moving the rdpmcl inline out of line.

Signed-off-by Andi Kleen <[email protected]>

---
 arch/x86/kernel/cpu/perf_event.c | 3 +++
 include/trace/events/msr.h       | 6 ++++++
 2 files changed, 9 insertions(+)

arch/x86/kernel/cpu/perf_event.c +3 -0

index e0dab5c..9644d95 100644
@@ -26,6 +26,8 @@
 #include <linux/bitops.h>
 #include <linux/device.h>
 
+#include <trace/events/msr.h>
+
 #include <asm/apic.h>
 #include <asm/stacktrace.h>
 #include <asm/nmi.h>

@@ -82,6 +84,7 @@ u64 x86_perf_event_update(struct perf_event *event)
 again:
 	prev_raw_count = local64_read(&hwc->prev_count);
 	rdpmcl(hwc->event_base_rdpmc, new_raw_count);
+	trace_rdpmc(hwc->event_base_rdpmc, new_raw_count, 0);
 
 	if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
 					new_raw_count) != prev_raw_count)

include/trace/events/msr.h +6 -0

index e1677e8..4fa81b5 100644
@@ -40,6 +40,12 @@ DEFINE_EVENT(msr_trace_class, write_msr,
 	     TP_ARGS(msr, val, failed)
 );
 
+DEFINE_EVENT(msr_trace_class, rdpmc,
+	     TP_PROTO(unsigned msr, u64 val, int failed),
+	     TP_ARGS(msr, val, failed)
+);
+
+
 #endif /* _TRACE_MSR_H */
 
 /* This part must be outside protection */

Signature · 1 line

1.9.3