Messages in this thread | | | Date | Thu, 6 Feb 2020 10:38:59 +0000 | From | Will Deacon <> | Subject | Re: [PATCH 2/2] perf/arm64: Allow per-task kernel breakpoints |
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On Tue, Feb 04, 2020 at 03:56:24AM +0530, Bhupesh Sharma wrote: > commit 478fcb2cdb23 ("arm64: Debugging support") disallowed per-task > kernel breakpoints on arm64 since these would have potentially > complicated the stepping code. > > However, we now have several use-cases (for e.g. perf) which require > per-task address execution h/w breakpoint to be exercised/set on arm64:
To be honest, the perf interface to hw_breakpoint is an abomination and I think we should remove it entirely for arm64. It's flakey, complicated, adds code to context-switch and reduces the capabilities available to ptrace.
> For e.g. we can set address execution h/w breakpoints, using the > format prescribed by 'perf-list' command: > mem:<addr>[/len][:access] [Hardware breakpoint] > > Without this patch, currently 'perf stat -e' reports that per-task > address execution h/w breakpoints are 'not supported' on arm64. See > below: > > $ TEST_FUNC="vfs_read" > $ ADDR=0x`cat /proc/kallsyms | grep -P "\\s$TEST_FUNC\$" | cut -f1 -d' '` > $ perf stat -e mem:$ADDR:x -x';' -- cat /proc/cpuinfo > /dev/null > > <not supported>;;mem:0xffff00001031dd68:x;0;100.00;; > > After this patch, this use-case can be supported: > > $ TEST_FUNC="vfs_read" > $ ADDR=0x`cat /proc/kallsyms | grep -P "\\s$TEST_FUNC\$" | cut -f1 -d' '` > $ perf stat -e mem:$ADDR:x -x';' -- cat /proc/cpuinfo > /dev/null > > 5;;mem:0xfffffe0010361d20:x;912200;100.00;; > > Cc: Mark Rutland <mark.rutland@arm.com> > Cc: Will Deacon <will@kernel.org> > Cc: Catalin Marinas <catalin.marinas@arm.com> > Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org> > Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com> > --- > arch/arm64/kernel/hw_breakpoint.c | 7 ------- > 1 file changed, 7 deletions(-) > > diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c > index 0b727edf4104..c28f04e02845 100644 > --- a/arch/arm64/kernel/hw_breakpoint.c > +++ b/arch/arm64/kernel/hw_breakpoint.c > @@ -562,13 +562,6 @@ int hw_breakpoint_arch_parse(struct perf_event *bp, > hw->address &= ~alignment_mask; > hw->ctrl.len <<= offset; > > - /* > - * Disallow per-task kernel breakpoints since these would > - * complicate the stepping code. > - */ > - if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target) > - return -EINVAL; > -
Sorry, but this is broken; the check is there for a reason, not just for fun!
Look at how the step handler toggles the bp registers.
Will
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