Re: [PATCH 4/9] include/hw: introduce CPU_MAX_NEGATIVE_ENV_OFFSET

[Richard Henderson]

On 6/30/23 14:25, Anton Johansson wrote:
> For reasons related to code-generation quality, the offset of
> CPUTLBDescFast and IcountDecr from CPUArchState needs to fit within
> 11 bits of displacement (arm[32|64] and riscv addressing modes).
>
> This commit introduces a new constant to store the maximum allowed
> negative offset, so it can be statically asserted to hold later on.
>
> Signed-off-by: Anton Johansson <anjo@rev.ng>
> ---
>   include/hw/core/cpu.h | 11 +++++++++++
>   1 file changed, 11 insertions(+)
>
> diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
> index c226d7263c..0377f74d48 100644
> --- a/include/hw/core/cpu.h
> +++ b/include/hw/core/cpu.h
> @@ -259,6 +259,17 @@ struct qemu_work_item;
>   
>   #define CPU_UNSET_NUMA_NODE_ID -1
>   
> +/*
> + * For reasons related to code-generation quality the fast path
> + * CPUTLBDescFast array and IcountDecr fields need to be located within a
> + * small negative offset of CPUArchState.  This requirement comes from
> + * host-specific addressing modes of arm[32|64] and riscv which uses 12-
> + * and 11 bits of displacement respectively.
> + */
> +#define CPU_MIN_DISPLACEMENT_BITS 11
> +#define CPU_MAX_NEGATIVE_ENV_OFFSET \
> +    (-(1 << CPU_MIN_DISPLACEMENT_BITS))

You'd want 6 bits, for AArch64 LDP (7-bit signed immediate).


r~