Re: [RFC PATCH] gdb/gic: expose cpu_index via MxTxAttrs

[Peter Maydell]

On Tue, 12 Apr 2022 at 11:45, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> When accessing HW via the gdbstub we can't easily figure out what the
> cpu_index is. The canonical case is current_cpu but for some cases
> that will be NULL. For debug accesses we can overload requester_id and
> make the GIC a bit smarter about fishing that out.
>
> [AJB: very much a PoC hack for now but interested if this makes sense.
> We could encode cpu_index in another field but that would grow
> MxTxAttrs even more.]
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/124
> ---
>  include/exec/memattrs.h |  2 +-
>  hw/core/cpu-sysemu.c    | 15 +++++++++++----
>  hw/intc/arm_gic.c       | 33 +++++++++++++++++++--------------
>  3 files changed, 31 insertions(+), 19 deletions(-)
>
> diff --git a/include/exec/memattrs.h b/include/exec/memattrs.h
> index 9fb98bc1ef..1333a34cb3 100644
> --- a/include/exec/memattrs.h
> +++ b/include/exec/memattrs.h
> @@ -43,7 +43,7 @@ typedef struct MemTxAttrs {
>       * (see MEMTX_ACCESS_ERROR).
>       */
>      unsigned int memory:1;
> -    /* Requester ID (for MSI for example) */
> +    /* Requester ID (for MSI for example) or cpu_index for debug */
>      unsigned int requester_id:16;

If we want to provide a requester ID for memory transactions we
should provide it always, not just for debug. That way gic_get_current_cpu()
and similar code can unconditionally use requester_id and never needs
to look at current_cpu. (We would also need to figure out how we want
to parcel out requester_ids in the system, so that PCI requester IDs
don't clash with CPU requester IDs.)

-- PMM