Re: [Qemu-devel] [PATCH v4 11/21] target-arm: Don't mention PMU in debug feature register

[Peter Crosthwaite]

On Fri, Mar 7, 2014 at 5:32 AM, Peter Maydell <address@hidden> wrote:
> Suppress the ID_AA64DFR0_EL1 PMUVer field, even if the CPU specific
> value claims that it exists. QEMU doesn't currently implement it,
> and not advertising it prevents the guest from trying to use it
> and getting UNDEFs on unimplemented registers.
>
> Signed-off-by: Peter Maydell <address@hidden>

Reviewed-by: Peter Crosthwaite <address@hidden>

> ---
> This is arguably a hack, but otherwise Linux tries to prod
> half a dozen PMU sysregs.

Not really. I think sane self-identification trumps dummy feature
advertising. Although there is a consistency argument to be made, as
to whether you should also wipe-out any other features advertised by
this register and friends (e.g. should TraceVer be set?).

Regards,
Peter

> ---
>  target-arm/helper.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/target-arm/helper.c b/target-arm/helper.c
> index c18f1a6..e1672aa 100644
> --- a/target-arm/helper.c
> +++ b/target-arm/helper.c
> @@ -1929,7 +1929,12 @@ void register_cp_regs_for_features(ARMCPU *cpu)
>              { .name = "ID_AA64DFR0_EL1", .state = ARM_CP_STATE_AA64,
>                .opc0 = 3, .opc1 = 0, .crn = 0, .crm = 5, .opc2 = 0,
>                .access = PL1_R, .type = ARM_CP_CONST,
> -              .resetvalue = cpu->id_aa64dfr0 },
> +              /* We mask out the PMUVer field, beacuse we don't currently
> +               * implement the PMU. Not advertising it prevents the guest
> +               * from trying to use it and getting UNDEFs on registers we
> +               * don't implement.
> +               */
> +              .resetvalue = cpu->id_aa64dfr0 & ~0xf00 },
>              { .name = "ID_AA64DFR1_EL1", .state = ARM_CP_STATE_AA64,
>                .opc0 = 3, .opc1 = 0, .crn = 0, .crm = 5, .opc2 = 1,
>                .access = PL1_R, .type = ARM_CP_CONST,
> --
> 1.9.0
>
>