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x86/events/intel/ds: Fix bts_interrupt_threshold alignment

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commit 2c991e408d upstream.

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: c1961a4631 ("x86/events/intel/ds: Map debug buffers in cpu_entry_area")
Reported-and-tested-by: Markus T Metzger <[email protected]>
Signed-off-by: Hugh Dickins <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: [email protected] # v4.14+
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
This commit is contained in:
Hugh Dickins
2018-07-25 11:25:07 +02:00
committed by Greg Kroah-Hartman
parent b4108288ea
commit aa49e48232
1 changed files with 5 additions and 3 deletions
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arch/x86/events/intel/ds.c
+5 -3
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@@ -410,9 +410,11 @@ 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;
}