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[ Upstream commit0f4cd769c4] 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 commitdb98c5faf8("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> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: "Arnaldo Carvalho de Melo" <acme@kernel.org> Cc: <x86@kernel.org> Cc: Ingo Molnar <mingo@kernel.org> Cc: Ingo Molnar <mingo@redhat.com> Cc: Jiri Olsa <jolsa@redhat.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: "Borislav Petkov" <bp@alien8.de> Cc: Stephane Eranian <eranian@google.com> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> Cc: "Namhyung Kim" <namhyung@kernel.org> Cc: "H. Peter Anvin" <hpa@zytor.com> Link: https://lkml.kernel.org/r/20190826195730.30614-1-kim.phillips@amd.com Signed-off-by: Sasha Levin <sashal@kernel.org>
Linux kernel
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