Re: [RFC PATCH] target/arm: allow gdb to read ARM_CP_NORAW regs

["Alex Bennée" <alex.bennee@linaro.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Wed, 7 May 2025 at 17:58, Alex Bennée <alex.bennee@linaro.org> wrote:
>>
>> Before this we suppress all ARM_CP_NORAW registers being listed under
>> GDB. This includes useful registers like CurrentEL which gets tagged
>> as ARM_CP_NO_RAW because it is one of the ARM_CP_SPECIAL_MASK
>> registers. These are registers TCG can directly compute because we
>> have the information at compile time but until now with no readfn.
>>
>> Add a .readfn to return the CurrentEL and then loosen the restrictions
>> in arm_register_sysreg_for_feature to allow ARM_CP_NORAW registers to
>> be read if there is a readfn available.
>
> The primary use case for NO_RAW is "system instructions" like
> the TLB maintenance insns. These don't make sense to expose
> to a debugger.

I think we could re-think the logic:

    /*
     * By convention, for wildcarded registers only the first
     * entry is used for migration; the others are marked as
     * ALIAS so we don't try to transfer the register
     * multiple times. Special registers (ie NOP/WFI) are
     * never migratable and not even raw-accessible.
     */
    if (r2->type & ARM_CP_SPECIAL_MASK) {
        r2->type |= ARM_CP_NO_RAW;
    }

> If we want the gdbstub access to system registers to be
> more than our current "we provide the ones that are easy",
> then I think I'd like to see a bit more up-front analysis of
> what the gdbstub needs and whether we've got into a bit of
> a mess with our ARM_CP_* flags that we could straighten out.

Yeah - hence the RFC. CurrentEL is a super useful one to expose though
when you are debugging complex hypervisor setups.

>
> thanks
> -- PMM

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro