Re: [PATCH 2/3] target/arm: provide RAZ/WI stubs for more DCC registers

[Peter Maydell <peter.maydell@linaro.org>]

On Thu, 5 Jan 2023 at 22:13, Evgeny Iakovlev
<eiakovlev@linux.microsoft.com> wrote:
>
> Qemu doesn't implement Debug Communication Channel, however when running
> Microsoft Hyper-V in software-emulated ARM64 as a guest, it tries to
> access some of the DCM registers during an EL2 context switch.

I've occasionally thought about implementing the DCC as something
the QEMU user could connect to a QEMU chardev. But that would be
a lot of faff for no very obvious benefit, so making these registers
RAZ makes sense for now.

> Provide RAZ/WI stubs for OSDTRRX_EL1, OSDTRTX_EL1 and OSECCR_EL1
> registers in the same way the rest of DCM is currently done. Do
> account for access traps though with access_tda.

OSECCR_EL1 isn't part of DCC; it's a different bit of the external
debug interface.

> Signed-off-by: Evgeny Iakovlev <eiakovlev@linux.microsoft.com>
> ---
>  target/arm/debug_helper.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/target/arm/debug_helper.c b/target/arm/debug_helper.c
> index b244e146e2..2a7c3d7e38 100644
> --- a/target/arm/debug_helper.c
> +++ b/target/arm/debug_helper.c
> @@ -673,6 +673,18 @@ static const ARMCPRegInfo debug_cp_reginfo[] = {
>        .opc0 = 2, .opc1 = 3, .crn = 0, .crm = 1, .opc2 = 0,
>        .access = PL0_R, .accessfn = access_tda,
>        .type = ARM_CP_CONST, .resetvalue = 0 },
> +    { .name = "OSDTRRX_EL1", .state = ARM_CP_STATE_BOTH, .cp = 14,
> +      .opc0 = 2, .opc1 = 0, .crn = 0, .crm = 0, .opc2 = 2,
> +      .access = PL1_RW, .accessfn = access_tda,
> +      .type = ARM_CP_CONST, .resetvalue = 0 },
> +    { .name = "OSDTRTX_EL1", .state = ARM_CP_STATE_BOTH, .cp = 14,
> +      .opc0 = 2, .opc1 = 0, .crn = 0, .crm = 3, .opc2 = 2,
> +      .access = PL1_RW, .accessfn = access_tda,
> +      .type = ARM_CP_CONST, .resetvalue = 0 },
> +    { .name = "OSECCR_EL1", .state = ARM_CP_STATE_BOTH, .cp = 14,
> +      .opc0 = 2, .opc1 = 0, .crn = 0, .crm = 6, .opc2 = 2,
> +      .access = PL1_RW, .accessfn = access_tda,
> +      .type = ARM_CP_CONST, .resetvalue = 0 },

A brief comment or two here would be nice.

Otherwise
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM