Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for constraints accounting

[Will Deacon <will@kernel.org>]

On Thu, Jul 21, 2022 at 05:22:07PM +0100, Mark Rutland wrote:
> Hi Marco,
> 
> [adding Will]
> 
> On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote:
> > Add KUnit test for hw_breakpoint constraints accounting, with various
> > interesting mixes of breakpoint targets (some care was taken to catch
> > interesting corner cases via bug-injection).
> > 
> > The test cannot be built as a module because it requires access to
> > hw_breakpoint_slots(), which is not inlinable or exported on all
> > architectures.
> > 
> > Signed-off-by: Marco Elver <elver@google.com>
> 
> As mentioned on IRC, I'm seeing these tests fail on arm64 when applied atop
> v5.19-rc7:
> 
> | TAP version 14
> | 1..1
> |     # Subtest: hw_breakpoint
> |     1..9
> |     ok 1 - test_one_cpu
> |     ok 2 - test_many_cpus
> |     # test_one_task_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 3 - test_one_task_on_all_cpus
> |     # test_two_tasks_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 4 - test_two_tasks_on_all_cpus
> |     # test_one_task_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 5 - test_one_task_on_one_cpu
> |     # test_one_task_mixed: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 6 - test_one_task_mixed
> |     # test_two_tasks_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 7 - test_two_tasks_on_one_cpu
> |     # test_two_tasks_on_one_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 8 - test_two_tasks_on_one_all_cpus
> |     # test_task_on_all_and_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
> |     Expected IS_ERR(bp) to be false, but is true
> |     not ok 9 - test_task_on_all_and_one_cpu
> | # hw_breakpoint: pass:2 fail:7 skip:0 total:9
> | # Totals: pass:2 fail:7 skip:0 total:9
> 
> ... which seems to be becasue arm64 currently forbids per-task
> breakpoints/watchpoints in hw_breakpoint_arch_parse(), where we have:
> 
>         /*
>          * Disallow per-task kernel breakpoints since these would
>          * complicate the stepping code.
>          */
>         if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
>                 return -EINVAL;
> 
> ... which has been the case since day one in commit:
> 
>   478fcb2cdb2351dc ("arm64: Debugging support")
> 
> I'm not immediately sure what would be necessary to support per-task kernel
> breakpoints, but given a lot of that state is currently per-cpu, I imagine it's
> invasive.

I would actually like to remove HW_BREAKPOINT completely for arm64 as it
doesn't really work and causes problems for other interfaces such as ptrace
and kgdb.

Will