Re: [Qemu-arm] [PATCH v3] target/arm: generate a custom MIDR for -cpu max

[Peter Maydell]

On Fri, 26 Jul 2019 at 10:48, Alex Bennée <address@hidden> wrote:
>
> While most features are now detected by probing the ID_* registers
> kernels can (and do) use MIDR_EL1 for working out of they have to
> apply errata. This can trip up warnings in the kernel as it tries to
> work out if it should apply workarounds to features that don't
> actually exist in the reported CPU type.
>
> Avoid this problem by synthesising our own MIDR value.
>
> Signed-off-by: Alex Bennée <address@hidden>
>
> ---
> v2
>   - don't leak QEMU version into ID reg
> v3
>   - move comment into one block
>   - explicit setting of more fields
> ---
>  target/arm/cpu.h   |  6 ++++++
>  target/arm/cpu64.c | 17 +++++++++++++++++
>  2 files changed, 23 insertions(+)
>
> diff --git a/target/arm/cpu.h b/target/arm/cpu.h
> index 94c990cddbd..67f2af0e169 100644
> --- a/target/arm/cpu.h
> +++ b/target/arm/cpu.h
> @@ -1611,6 +1611,12 @@ FIELD(V7M_FPCCR, ASPEN, 31, 1)
>  /*
>   * System register ID fields.
>   */
> +FIELD(MIDR_EL1, REVISION, 0, 4)
> +FIELD(MIDR_EL1, PARTNUM, 4, 12)
> +FIELD(MIDR_EL1, ARCHITECTURE, 16, 4)
> +FIELD(MIDR_EL1, VARIANT, 20, 4)
> +FIELD(MIDR_EL1, IMPLEMENTER, 24, 8)
> +
>  FIELD(ID_ISAR0, SWAP, 0, 4)
>  FIELD(ID_ISAR0, BITCOUNT, 4, 4)
>  FIELD(ID_ISAR0, BITFIELD, 8, 4)
> diff --git a/target/arm/cpu64.c b/target/arm/cpu64.c
> index 1901997a064..f99008d7da1 100644
> --- a/target/arm/cpu64.c
> +++ b/target/arm/cpu64.c
> @@ -296,6 +296,23 @@ static void aarch64_max_initfn(Object *obj)
>          uint32_t u;
>          aarch64_a57_initfn(obj);
>
> +        /*
> +         * Reset MIDR so the guest doesn't mistake our 'max' CPU type for a 
> real
> +         * one and try to apply errata workarounds or use impdef features we
> +         * don't provide.
> +         * An IMPLEMENTER field of 0 means "reserved for software use";
> +         * ARCHITECTURE must be 0xf indicating "v7 or later, check ID 
> registers
> +         * to see which features are present";
> +         * the VARIANT, PARTNUM and REVISION fields are all implementation
> +         * defined and we choose to set VARIANT and leave the versions at 
> zero.
> +         */
> +        t = FIELD_DP64(0, MIDR_EL1, IMPLEMENTER, 0);
> +        t = FIELD_DP64(t, MIDR_EL1, ARCHITECTURE, 0xf);
> +        t = FIELD_DP64(t, MIDR_EL1, PARTNUM, 'Q');
> +        t = FIELD_DP64(t, MIDR_EL1, VARIANT, 0);
> +        t = FIELD_DP64(t, MIDR_EL1, REVISION, 0);

Comment says we set VARIANT and leave the "versions" (what are they?)
at 0, but the code says we set PARTNUM...

With the comment text fixed to match the code,
Reviewed-by: Peter Maydell <address@hidden>

thanks
-- PMM