Re: [PATCH] arm64: hw_breakpoint: don't clear debug registers in halt mode

From: Will Deacon <will@kernel.org>
To: tingwei@codeaurora.org
Cc: Mark Rutland <mark.rutland@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: hw_breakpoint: don't clear debug registers in halt mode
Date: Tue, 21 Apr 2020 08:10:45 +0100	[thread overview]
Message-ID: <20200421071043.GA14844@willie-the-truck> (raw)
In-Reply-To: <20b8c92a16db07978dfcb8cdf3b0e185@codeaurora.org>

On Tue, Apr 21, 2020 at 11:49:11AM +0800, tingwei@codeaurora.org wrote:
> 在 2020-03-31 19:45，Will Deacon 写道：
> > On Tue, Mar 31, 2020 at 07:33:38PM +0800, tingwei@codeaurora.org wrote:
> > > 在 2020-03-31 15:41，Will Deacon 写道：
> > > > On Tue, Mar 31, 2020 at 10:39:42AM +0800, tingwei@codeaurora.org wrote:
> > > > > For reset the registers after "debug_enabled" is toggled, I'm
> > > > > thinking if
> > > > > we are adding unnecessary complexity here.If we take that approach, we
> > > > > will
> > > > > hook "debug_enabled" interface and use smp_call_function_single() to
> > > > > call
> > > > > hw_breakpoint_reset() on each CPU. Wait for all CPUs' execution done
> > > > > and
> > > > > change "debug_enabled". External debugger would clear the
> > > > > breakpoints when
> > > > > it detaches the device and restores its breakpoints when attaches the
> > > > > device.
> > > > > Assume debug_enabled is changed to one after external debugger
> > > > > detaches
> > > > > the
> > > > > device. Debugger would already clear the breakpoint registers. If
> > > > > debgger
> > > > > is
> > > > > still attached, there's nothing Kernel can do to stop it
> > > > > restores/programs
> > > > > the breakpoint registers.
> > > > >
> > > > > What do you think of this?
> > > >
> > > > It's all a bit of a mess. Looking at it some more, why can't the
> > > > external
> > > > debugger simply trap access to the debug registers using EDSCR.TDA? That
> > > > way, we don't have to change anything in the kernel.
> > > >
> > > 
> > > External debugger has the function to trap access to debug registers
> > > now.
> > > What do we expect debugger to do after core is stopped? Skip that msr
> > > instruction and continue to run?
> > 
> > The nicest thing to do would probably be to record all the accesses made
> > by the OS so that it can emulate reads and replay writes when external
> > debugging is over. Given that you'd still be expecting to pass
> > "nodebugmon",
> > the emulation should be pretty straightforward, I think.
> > 
> 
> To provide an update on this, I've worked with external debugger vendor on
> this.
> Now external debugger can trap the write to debug registers and ignore the
> write.
> This is the first step.

Thanks for the update! Please let us know if you run into any unforeseen
problems.

Will

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