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The ARMv9.2 architecture introduces the optional Branch Record Buffer
Extension (BRBE), which records information about branches as they are
executed into set of branch record registers. BRBE is similar to x86's
Last Branch Record (LBR) and PowerPC's Branch History Rolling Buffer
(BHRB).
BRBE supports filtering by exception level and can filter just the
source or target address if excluded to avoid leaking privileged
addresses. The h/w filter would be sufficient except when there are
multiple events with disjoint filtering requirements. In this case, BRBE
is configured with a union of all the events' desired branches, and then
the recorded branches are filtered based on each event's filter. For
example, with one event capturing kernel events and another event
capturing user events, BRBE will be configured to capture both kernel
and user branches. When handling event overflow, the branch records have
to be filtered by software to only include kernel or user branch
addresses for that event. In contrast, x86 simply configures LBR using
the last installed event which seems broken.
It is possible on x86 to configure branch filter such that no branches
are ever recorded (e.g. -j save_type). For BRBE, events with a
configuration that will result in no samples are rejected.
Recording branches in KVM guests is not supported like x86. However,
perf on x86 allows requesting branch recording in guests. The guest
events are recorded, but the resulting branches are all from the host.
For BRBE, events with branch recording and "exclude_host" set are
rejected. Requiring "exclude_guest" to be set did not work. The default
for the perf tool does set "exclude_guest" if no exception level
options are specified. However, specifying kernel or user events
defaults to including both host and guest. In this case, only host
branches are recorded.
BRBE can support some additional exception branch types compared to
x86. On x86, all exceptions other than syscalls are recorded as IRQ.
With BRBE, it is possible to better categorize these exceptions. One
limitation relative to x86 is we cannot distinguish a syscall return
from other exception returns. So all exception returns are recorded as
ERET type. The FIQ branch type is omitted as the only FIQ user is Apple
platforms which don't support BRBE. The debug branch types are omitted
as there is no clear need for them.
BRBE records are invalidated whenever events are reconfigured, a new
task is scheduled in, or after recording is paused (and the records
have been recorded for the event). The architecture allows branch
records to be invalidated by the PE under implementation defined
conditions. It is expected that these conditions are rare.
Cc: Catalin Marinas <catalin.marinas@arm.com>
Co-developed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Co-developed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: James Clark <james.clark@linaro.org>
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
tested-by: Adam Young <admiyo@os.amperecomputing.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20250611-arm-brbe-v19-v23-4-e7775563036e@kernel.org
[will: Fix sparse warnings about mixed declarations and code.
Fix C99 comment syntax.]
Signed-off-by: Will Deacon <will@kernel.org>
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