Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220704150514.48816-1-elver@google.com> <20220704150514.48816-2-elver@google.com>
 <Ytl9L0Zn1PVuL1cB@FVFF77S0Q05N.cambridge.arm.com> <20220722091044.GC18125@willie-the-truck>
 <CACT4Y+ZOXXqxhe4U3ZtQPCj2yrf6Qtjg1q0Kfq8+poAOxGgUew@mail.gmail.com>
 <20220722101053.GA18284@willie-the-truck> <CACT4Y+Z0imEHF0jM-f-uYdpfSpfzMpa+bFZfPeQW1ECBDjD9fA@mail.gmail.com>
 <20220722110305.GA18336@willie-the-truck>
In-Reply-To: <20220722110305.GA18336@willie-the-truck>
From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Fri, 22 Jul 2022 15:41:38 +0200
Message-ID: <CACT4Y+aLiNNt3ESZUKHT9U8duN-TMK561nC7Htx9y3R7afCV4g@mail.gmail.com>
Subject: Re: [PATCH v3 01/14] perf/hw_breakpoint: Add KUnit test for
 constraints accounting
To:     Will Deacon <will@kernel.org>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Marco Elver <elver@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev@lists.ozlabs.org, linux-perf-users@vger.kernel.org,
        x86@kernel.org, linux-sh@vger.kernel.org,
        kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, 22 Jul 2022 at 13:03, Will Deacon <will@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).

x86 supports this.
There is that attr.exclude_kernel flag that requires special permissions:
https://elixir.bootlin.com/linux/v5.19-rc7/source/kernel/events/core.c#L12061
https://elixir.bootlin.com/linux/v5.19-rc7/source/kernel/events/core.c#L9323
But if I understand correctly, it only filters out delivery, the HW
breakpoint fires even if attr.exclude_kernel is set.

We also wanted to relax this permission check somewhat:
https://lore.kernel.org/all/20220601093502.364142-1-elver@google.com/

Yes, if the kernel maps the page at a different virtual address, then
the breakpoint won't fire I think.
Don't know what are the issues with firmware/KVM.