Support for unaligned watchpoints in arm/arm64

[Pavel Labath]

On 7 June 2016 at 08:17, Pratyush Anand <panand at redhat.com> wrote:
> On 07/06/2016:12:25:20 PM, Pratyush Anand wrote:
>> On 31/05/2016:01:38:27 PM, Will Deacon wrote:
>> > On Thu, May 26, 2016 at 05:04:46PM +0100, Pavel Labath wrote:
>> > > Hello all,
>> >
>> > Hi Pavel,
>> >
>> > > I've been wondering if there are any plans about adding support for
>> > > unaligned watchpoints to the kernel. It seems quite a shame that
>> > > applications are not able not use them, even though the hardware
>> > > should support that feature.
>> >
>> > I'm actually coming round to the idea of ditching the perf hw_breakpoint
>> > mechanism entirely and simply writing a ptrace back-end that can expose
>> > the hardware features directly to userspace. The two issues with this
>> > are:
>> >
>> >   (1) It's a fair amount of work
>>
>> So, by the time this new interface would come, probably we can consider a fixup
>> like following to resolve this issue at hand. Probably, things should work by
>> just allowing hw_breakpoint.c to pass checks for unaligned offset when it is a
>> WATCHPOINT, no?
>
> Sorry, this is not going to work.
> There could be some BAS values where few consecutive LSBs are 0s.
>

Thank you for the replies.

I would definitely welcome a more immediate fix to this issue,
although I can appreciate the reluctance to add ugly fixups to make
this work.

Pratyush, building on your proposed fix, I think we could extend it to
make it work for watching values which are not word-aligned by
tweaking  perf_event_attr.bp_addr field. E.g. if the BAS value is
00111100 then we can set bp_addr to "aligned_addr + 2" and bp_len to
4. From an interface point of view I think this would be a nice
solution. ptrace userspace user would set these breakpoints just like
it would do when programming a real register, and the contents of the
struct perf_event_attr will make perfect sense: "we have set a
watchpoint at bp_addr with length bp_len". The only problem is that
the code for converting between the perf_event_attr and hardware
register representations will be quite ugly. Also, I don't know if
this will cause a problem with any other users of perf_event_attr if
they expect bp_addr field to be word-aligned.

If you would be willing to consider such a patch, I can take a stab at
writing it. What do you think?

regards,
pl