Re: Plugin Register Accesses

[Alex Bennée]

Aaron Lindsay <aaron@os.amperecomputing.com> writes:

> On Dec 08 12:17, Alex Bennée wrote:
>> Aaron Lindsay <aaron@os.amperecomputing.com> writes:
>> 
>> > I'm trying to migrate to using the new plugin interface. I see the
>> > following in include/qemu/qemu-plugin.h:
>> >
>> >> enum qemu_plugin_cb_flags {
>> >>     QEMU_PLUGIN_CB_NO_REGS, /* callback does not access the CPU's regs */
>> >>     QEMU_PLUGIN_CB_R_REGS,  /* callback reads the CPU's regs */
>> >>     QEMU_PLUGIN_CB_RW_REGS, /* callback reads and writes the CPU's regs */
>> >> };
>> >
>> > But I don't see a way to access registers in callbacks. Am I missing
>> > something?
>> 
>> No - while those symbols do inform the TCG to not try and optimise
>> the register file we don't yet have an API for the plugins for reading
>> (or writing) the CPU registers.
>> 
>> There has been discussion about this before, I'll quote what I said
>> off-list to someone else who asked:
>> 
>>   > Has there been any clarification or softening of the position that 
>>   > exposing register and memory contents to the QEMU plugin would provide a 
>>   > way to circumvent the GPL of QEMU?
>> 
>>   I don't think implementing read only access would be a problem and
>>   should probably be a first step anyway.
>
> That seems reasonable to me. For the time being, at least, I am most
> interested in read-only access.
>
>>   For registers I think there needs to be some re-factoring of QEMU's
>>   internals to do it cleanly. Currently we have each front-end providing
>>   hooks to the gdbstub as well as building up their own regid and xml to
>>   be consumed by it. We probably want a architectural neutral central
>>   repository that the front ends can register their registers (sic) and
>>   helpers with. This would then provide hooks for gdbstub to cleanly
>>   generate XML as well as an interface point for the plugin infrastructure
>>   (and probably whatever the HMP uses as well).
>
> In a previous incarnation, I was proxying calls to the plugin API
> directly through to gdb_read_register() in gdbstub.c and therefore using
> gdb as the point of commonality. I'm not saying it's ideal but... it
> works? One downside is that you have to know 'out-of-band' which integer
> value corresponds to the register you want to query for your
> architecture, though it hasn't been a significant issue for me.

Certainly workable for a private branch but I don't want to merge
anything like that upstream. As far as I can see there are a number of
consumers of register information:

  - plugins
  - gdbstub
  - monitor (info registers)
  - -d LOG_CPU logging

so rather than have them all have their hooks into every front-end I can
see a case for consolidation.

For the plugin case providing an introspection helper to get a handle on
the register makes sense and would be less painful than teachning
plugins about gdb regids which can and do move around with new system
registers.

  qemu_plugin_reg_t *handle = qemu_plugin_find_register("x2")

if we document the handle as usable across calls this can be done on
start-up. Then it would be:

  uint64_t val = qemu_plugin_read_register(cpu_index, handle);

>>   Memory is a little trickier because you can't know at any point if a
>>   given virtual address is actually mapped to real memory. The safest way
>>   would be to extend the existing memory tracking code to save the values
>>   saved/loaded from a given address. However if you had register access
>>   you could probably achieve the same thing after the fact by examining
>>   the opcode and pulling the values from the registers.
>
> What if memory reads were requested by `qemu_plugin_hwaddr` instead of
> by virtual address? `qemu_plugin_get_hwaddr()` is already exposed, and I
> would expect being able to successfully get a `qemu_plugin_hwaddr` in a
> callback would mean it is currently mapped. Am I overlooking
> something?

We can't re-run the transaction - there may have been a change to the
memory layout that instruction caused (see tlb_plugin_lookup and the
interaction with io_writex). However I think we can expand the options
for memory instrumentation to cache the read or written value.

> I think I might actually prefer a plugin memory access interface be in
> the physical address space - it seems like it might allow you to get
> more mileage out of one interface without having to support accesses by
> virtual and physical address separately.
>
> Or, even if that won't work for whatever reason, it seems reasonable for
> a plugin call accessing memory by virtual address to fail in the case
> where it's not mapped. As long as that failure case is well-documented
> and easy to distinguish from others within a plugin, why not?

Hmmm I'm not sure - I don't want to expose internal implementation
details to the plugins because we don't want plugins to rely on them.

>
> -Aaron


-- 
Alex Bennée