From 2c991e408df6a407476dbc453d725e1e975479e7 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Sat, 14 Jul 2018 12:58:07 -0700 Subject: [PATCH] x86/events/intel/ds: Fix bts_interrupt_threshold alignment Markus reported that BTS is sporadically missing the tail of the trace in the perf_event data buffer: [decode error (1): instruction overflow] shown in GDB; and bisected it to the conversion of debug_store to PTI. A little "optimization" crept into alloc_bts_buffer(), which mistakenly placed bts_interrupt_threshold away from the 24-byte record boundary. Intel SDM Vol 3B 17.4.9 says "This address must point to an offset from the BTS buffer base that is a multiple of the BTS record size." Revert "max" from a byte count to a record count, to calculate the bts_interrupt_threshold correctly: which turns out to fix problem seen. Fixes: c1961a4631da ("x86/events/intel/ds: Map debug buffers in cpu_entry_area") Reported-and-tested-by: Markus T Metzger Signed-off-by: Hugh Dickins Signed-off-by: Thomas Gleixner Cc: Peter Zijlstra Cc: Arnaldo Carvalho de Melo Cc: Alexander Shishkin Cc: Andi Kleen Cc: Dave Hansen Cc: Stephane Eranian Cc: stable@vger.kernel.org # v4.14+ Link: https://lkml.kernel.org/r/alpine.LSU.2.11.1807141248290.1614@eggly.anvils --- arch/x86/events/intel/ds.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c index 8a10a04..8cf03f1 100644 --- a/arch/x86/events/intel/ds.c +++ b/arch/x86/events/intel/ds.c @@ -408,9 +408,11 @@ static int alloc_bts_buffer(int cpu) ds->bts_buffer_base = (unsigned long) cea; ds_update_cea(cea, buffer, BTS_BUFFER_SIZE, PAGE_KERNEL); ds->bts_index = ds->bts_buffer_base; - max = BTS_RECORD_SIZE * (BTS_BUFFER_SIZE / BTS_RECORD_SIZE); - ds->bts_absolute_maximum = ds->bts_buffer_base + max; - ds->bts_interrupt_threshold = ds->bts_absolute_maximum - (max / 16); + max = BTS_BUFFER_SIZE / BTS_RECORD_SIZE; + ds->bts_absolute_maximum = ds->bts_buffer_base + + max * BTS_RECORD_SIZE; + ds->bts_interrupt_threshold = ds->bts_absolute_maximum - + (max / 16) * BTS_RECORD_SIZE; return 0; } -- 2.7.4