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

[Dmitry Vyukov <dvyukov@google.com>]

On Fri, 22 Jul 2022 at 11:10, Will Deacon <will@kernel.org> wrote:
> > [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 it be a localized removal of code that will be easy to revert in
future? Or will it touch lots of code here and there?
Let's say we come up with a very important use case for HW_BREAKPOINT
and will need to make it work on arm64 as well in future.