Re: [RFC PATCH] plugins: filter out the PC from the register list

[Peter Maydell]

On Tue, 5 Mar 2024 at 17:13, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> QEMU's handling of the program counter is special in so far as the
> translator avoids setting it whenever possible. As the PC is available
> at translation time lets avoid confusion by just filtering the program
> counter from the list of available registers.
>
> Update the documentation with some notes about the register access and
> calling out the behaviour of the PC.
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2208
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>  docs/devel/tcg-plugins.rst | 15 +++++++++++++++
>  plugins/api.c              |  5 +++++
>  2 files changed, 20 insertions(+)
>
> diff --git a/docs/devel/tcg-plugins.rst b/docs/devel/tcg-plugins.rst
> index 9cc09d8c3da..656df012e77 100644
> --- a/docs/devel/tcg-plugins.rst
> +++ b/docs/devel/tcg-plugins.rst
> @@ -151,6 +151,21 @@ Unsuccessful operations (i.e. faults) will not be 
> visible to memory
>  instrumentation although the execution side effects can be observed
>  (e.g. entering a exception handler).
>
> +Register Values
> ++++++++++++++++
> +
> +Callbacks registered with the ``QEMU_PLUGIN_CB_R_REGS`` flags can read
> +the current register values of the system. The plugin need to request
> +the list of available registers after a vcpu has initialised by
> +calling ``qemu_plugin_get_registers`` and using the supplied handle to
> +query the values when executing the callback.
> +
> +.. Note:: the program counter (PC) is not available through this
> +          interface but can be queried at translation time by using
> +          the ``qemu_plugin_insn_vaddr`` and
> +          ``qemu_plugin_insn_haddr`` on the instruction handle.
> +
> +
>  System Idle and Resume States
>  +++++++++++++++++++++++++++++
>
> diff --git a/plugins/api.c b/plugins/api.c
> index 8fa5a600ac3..fc3477acf2d 100644
> --- a/plugins/api.c
> +++ b/plugins/api.c
> @@ -448,6 +448,11 @@ static GArray *create_register_handles(GArray 
> *gdbstub_regs)
>              continue;
>          }
>
> +        /* skip the program counter */
> +        if (g_ascii_strncasecmp(grd->name, "pc", 2) == 0) {
> +            continue;
> +        }

Is the program counter always named "pc"? For instance
gdb-xml/microblaze-core.xml suggests it's named "rpc" there.
And on x86-64 I think from the gdb protocol point of view it
is named "rip" and the gdb "$pc" alias is handled on the gdb
side, but maybe I'm wrong.

Does the same problem apply to other registers whose value
we fix up after-the-fact via the restore_state_to_opc hook,
like the SPARC npc register, or the 32-bit Arm Thumb
IT bits in the CPSR?

thanks
-- PMM