When counting dispatched micro-ops with cnt_ctl=1, in order to prevent sample bias, IBS hardware preloads the least significant 7 bits of current count (IbsOpCurCnt) with random values, such that, after the interrupt is handled and counting resumes, the next sample taken will be slightly perturbed.

The current count bitfield is in the IBS execution control h/w register, alongside the maximum count field.

Currently, the IBS driver writes that register with the maximum count, leaving zeroes to fill the current count field, thereby overwriting the random bits the hardware preloaded for itself.

Fix the driver to actually retain and carry those random bits from the read of the IBS control register, through to its write, instead of overwriting the lower current count bits with zeroes.

Tested with:

perf record -c 100001 -e ibs_op/cnt_ctl=1/pp -a -C 0 taskset -c 0 <workload>

'perf annotate' output before:

15.70 65: addsd %xmm0,%xmm1 17.30 add $0x1,%rax 15.88 cmp %rdx,%rax je 82 17.32 72: test $0x1,%al jne 7c 7.52 movapd %xmm1,%xmm0 5.90 jmp 65 8.23 7c: sqrtsd %xmm1,%xmm0 12.15 jmp 65

'perf annotate' output after:

16.63 65: addsd %xmm0,%xmm1 16.82 add $0x1,%rax 16.81 cmp %rdx,%rax je 82 16.69 72: test $0x1,%al jne 7c 8.30 movapd %xmm1,%xmm0 8.13 jmp 65 8.24 7c: sqrtsd %xmm1,%xmm0 8.39 jmp 65

Tested on Family 15h and 17h machines.

Machines prior to family 10h Rev. C don't have the RDWROPCNT capability, and have the IbsOpCurCnt bitfield reserved, so this patch shouldn't affect their operation.

It is unknown why commit db98c5faf8cb ("perf/x86: Implement 64-bit counter support for IBS") ignored the lower 4 bits of the IbsOpCurCnt field; the number of preloaded random bits has always been 7, AFAICT.

Signed-off-by: Kim Phillips kim.phillips@amd.com Cc: Stephane Eranian eranian@google.com Cc: Peter Zijlstra peterz@infradead.org Cc: Ingo Molnar mingo@redhat.com Cc: Arnaldo Carvalho de Melo acme@kernel.org Cc: Alexander Shishkin alexander.shishkin@linux.intel.com Cc: Jiri Olsa jolsa@redhat.com Cc: Namhyung Kim namhyung@kernel.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Borislav Petkov bp@alien8.de Cc: "H. Peter Anvin" hpa@zytor.com Cc: x86@kernel.org Cc: stable@vger.kernel.org --- arch/x86/events/amd/ibs.c | 11 +++++++++-- arch/x86/include/asm/perf_event.h | 9 ++++++--- 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c index 62f317c9113a..f2625b4a5a8b 100644 --- a/arch/x86/events/amd/ibs.c +++ b/arch/x86/events/amd/ibs.c @@ -663,8 +663,15 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs) out: if (throttle) perf_ibs_stop(event, 0); - else - perf_ibs_enable_event(perf_ibs, hwc, period >> 4); + else { + period >>= 4; + + if ((ibs_caps & IBS_CAPS_RDWROPCNT) && + (*config & IBS_OP_CNT_CTL)) + period |= *config & IBS_OP_CUR_CNT_RAND; + + perf_ibs_enable_event(perf_ibs, hwc, period); + }

perf_event_update_userpage(event);

diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h index 1392d5e6e8d6..67d94696a1d6 100644 --- a/arch/x86/include/asm/perf_event.h +++ b/arch/x86/include/asm/perf_event.h @@ -259,9 +259,12 @@ struct pebs_lbr { #define IBS_FETCH_CNT 0xFFFF0000ULL #define IBS_FETCH_MAX_CNT 0x0000FFFFULL

-/* ibs op bits/masks */ -/* lower 4 bits of the current count are ignored: */ -#define IBS_OP_CUR_CNT (0xFFFF0ULL<<32) +/* ibs op bits/masks + * The lower 7 bits of the current count are random bits + * preloaded by hardware and ignored in software + */ +#define IBS_OP_CUR_CNT (0xFFF80ULL<<32) +#define IBS_OP_CUR_CNT_RAND (0x0007FULL<<32) #define IBS_OP_CNT_CTL (1ULL<<19) #define IBS_OP_VAL (1ULL<<18) #define IBS_OP_ENABLE (1ULL<<17)