[arm/hw_breakpoint] Detecting that hardware watchpoint support is unavailable

[Will Deacon]

On Fri, Jul 07, 2017 at 01:37:53PM +0100, Pavel Labath wrote:
> Hello Pratyush, Will,
> 
> I have an arm(32) chip, which has hardware debug support disabled, as
> far as I can tell (DBGDSCR.MDBGen reads as zero even after explicitly
> setting in).
> 
> The current behavior of the kernel on these chips is to return a
> non-zero number for the "number of supported watchpoint resources"
> when queried with ptrace and then fail at runtime (ENODEV) when the
> tracer attempts to set this breakpoint.
> 
> This does not seem like a particularly nice api, and I believe it
> would be better to just return zero for the watchpoint count
> initially, as we already know that setting the watchpoints will fail.
> 
> arch_hw_breakpoint_init() zeroes out the core_num_wrps when it detects
> that debug support is not present, however when we return resource
> info (ptrace_get_hbp_resource_info), we go through
> hw_breakpoint_slots(), which does not read these.
> 
> I think having ptrace_get_hbp_resource_info() read core_num_wrps would
> be more consistent because that's how we read maximum watchpoint
> length (arch_get_max_wp_len), which means we (correctly ?) return 0
> when hardware debug is not supported. In fact, that is how I presently
> detect that hardware debug is not supported (reading max_wp_len), but
> at this moment that seems more like an accident than a deliberate
> interface. Having num_wrps, num_brps zeroed out as well would make it
> clear that this is how it's intended to be used.
> 
> So, what do you think about a patch like that?

I don't want to expose hw_breakpoint internals to ptrace as long as we
go through the internal perf interface. The two options really are:

  1. Rip out the perf crap and integrate ptrace more tightly with
     hw_breakpoint.

  2. Fix hw_breakpoint_slots(...) to do what you want, although I seem
     to recall a chicken-and-egg problem there where perf insists on calling
     that function (hw_breakpoint_slots) early.

Will