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

[Will Deacon]

On Fri, Jul 22, 2022 at 12:31:45PM +0200, Dmitry Vyukov wrote:
> On Fri, 22 Jul 2022 at 12:11, Will Deacon <will at kernel.org> wrote:
> > > > > On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote:
> > > > > 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.
> >
> > My (rough) plan is to implement a lower-level abstraction for handling the
> > underlying hardware resources, so we can layer consumers on top of that
> > instead of funneling through hw_breakpoint. So if we figure out how to make
> > bits of hw_breakpoint work on arm64, then it should just go on top.
> >
> > The main pain point for hw_breakpoint is kernel-side {break,watch}points
> > and I think there are open design questions about how they should work
> > on arm64, particularly when considering the interaction with user
> > watchpoints triggering on uaccess routines and the possibility of hitting
> > a kernel watchpoint in irq context.
> 
> I see. Our main interest would be break/watchpoints on user addresses
> firing from both user-space and kernel (uaccess), so at least on irqs.

Interesting. Do other architectures report watchpoint hits on user
addresses from kernel uaccess? It feels like this might be surprising to
some users, and it opens up questions about accesses using different virtual
aliases (e.g. via GUP) or from other entities as well (e.g. firmware,
KVM guests, DMA).

Will