Re: [RFC] tcg plugin: Additional plugin interface

[Alex Bennée]

Min-Yih Hsu <minyihh@uci.edu> writes:

> Hi Alex and QEMU developers,
>
> Recently I was working with the TCG plugin. I found that 
> `qemu_plugin_cb_flags` seems to reserve the functionality to
> read / write CPU register state, I'm wondering if you can share some
> roadmap or thoughts on this feature?

I think reading the CPU register state is certainly on the roadmap,
writing registers presents a more philosophical question of if it opens
the way to people attempting a GPL bypass via plugins. However reading
registers would certainly be a worthwhile addition to the API.

> Personally I see reading the CPU register state as (kind of) low-hanging 
> fruit. The most straightforward way to implement
> it will be adding another function that can be called by insn_exec callbacks 
> to read (v)CPU register values. What do you
> think about this?

It depends on your definition of low hanging fruit ;-)

Yes the implementation would be a simple helper which could be called
from a callback - I don't think we need to limit it to just insn_exec. I
think the challenge is proving a non-ugly API that works cleanly across
all the architectures. I'm not keen on exposing arbitrary gdb register
IDs to the plugins.

There has been some discussion previously on the list which is probably
worth reviewing:

  Date: Mon, 7 Dec 2020 16:03:24 -0500
  From: Aaron Lindsay <aaron@os.amperecomputing.com>
  Subject: Plugin Register Accesses
  Message-ID: <X86YnHhHMpQBr2/G@strawberry.localdomain>

But in short I think we need a new subsystem in QEMU where frontends can
register registers (sic) and then provide a common API for various
users. This common subsystem would then be the source of data for:

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

If you are interested in tackling such a project I'm certainly happy to
provide pointers and review.

>
> Thank you
> -Min


-- 
Alex Bennée