[PATCH 1/8] gdb: Add supply_pseudo_pc hook to gdbarch.

[Marcin Kościelnicki]

On 16/02/16 19:27, Ulrich Weigand wrote:
> Marcin Kościelnicki wrote:
>
>> I don't think generalizing that function to arbitrary pseudo registers
>> is a good idea - there's already a pseudo_register_write, which covers
>> writing them.  The only reason I'm not using it in tracepoint.c is
>> that there's no "current register state" to write over, I'm making
>> a new one from scratch.
>
> Well, strictly speaking there is already a "current register state"
> since tracefile_fetch_registers just initialized all registers to
> the "unavailable" state.  At this point they could be fetched (and
> would read all zeros).
>
> So in principle you could just use regcache_write_pc at this point.
> The reason this doesn't work is a different one: that routine would
> attempt to *update target state*, i.e. it would call to the target's
> to_store_registers method.  However, the tracefile target only
> provides a to_fetch_registers method, and no to_store_registers,
> and the default target to_store_registers method just errors out.
>
> You *could* add a to_store_registers tracefile target method that
> would simply ignore writes (in effect, making the tracefile regcache
> modifyable without any underlying "real" target); then you could
> just use regcache_write_pc.
>
> However, this would not address the fact that there are potentially
> target-specific settings beyond a pure PC value that must be
> present in order to make a "synthetic" regcache at least minimally
> consistent.  This includes e.g. the address mode bit on 31-bit.
> Note that a default value of 0 isn't really valid for the PSWM
> register either, even though probably nobody checks.  But there
> are a number of bits that have to be set to make it a valid
> user-space PSW (e.g. interrupts and address translation must be
> enabled etc.).  If you'd ever try to load the PSW back to the
> hardware via ptrace, a 0 PSWM would get rejected.
>
>> Perhaps a better generalization would be to instead make a hook that's
>> supposed to provide a whole regcache based on the tracepoint location,
>> not just PC?  I can imagine a few uses for that:
>>
>> - on machines like ARM where you have several instruction encodings,
>>    you could also supply whatever registers determine the encoding
>>    (arm vs thumb) based on the location.
>> - if you can determine from the debug information that the function
>>    keeps GOT pointer / literal pool pointer / whatever in some register
>>    at the tracepoint location, you can also guess the contents of these
>>    (although that's probably not all that useful, and can be a lie
>>    if something is horribly buggy).
>
> Given the above, I therefore agree that this is really probably
> the preferable level of abstraction here.  You want a gdbarch hook
> to synthesize a minimally-valid regcache given only information
> statically known about the tracepoint.  Whether this hook gets
> just the PC or a full tracepoint structure is probably not so
> important right now; this could still be modified if necessary.
>
> I'd also suggest that this hook doesn't get a predicate, but a
> default implementation, which does the current handling of just
> setting a real PC register.  If overridden by a gdbarch, the
> platform can do anything that's necessary there instead.
>
> Bye,
> Ulrich
>

Sounds good, I'll do that.