Re: [PATCH v3 17/51] target/arm: Add cpu properties for SME

[Peter Maydell]

On Mon, 20 Jun 2022 at 19:08, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> Mirror the properties for SVE.  The main difference is
> that any arbitrary set of powers of 2 may be supported,
> and not the stricter constraints that apply to SVE.

> +SME CPU Property Examples
> +-------------------------
> +
> +  1) Disable SME::
> +
> +     $ qemu-system-aarch64 -M virt -cpu max,sme=off
> +
> +  2) Implicitly enable all vector lengths for the ``max`` CPU type::
> +
> +     $ qemu-system-aarch64 -M virt -cpu max
> +
> +  3) Only enable the 256-bit vector length::
> +
> +     $ qemu-system-aarch64 -M virt -cpu max,sme256=on
> +
> +  3) Enable the 256-bit and 1024-bit vector lengths::
> +
> +     $ qemu-system-aarch64 -M virt -cpu max,sme256=on,sme1024=on
> +
> +  4) Disable the 512-bit vector length.  This results in all the other
> +     lengths supported by ``max`` defaulting to enabled
> +     (128, 256, 1024 and 2048)::
> +
> +     $ qemu-system-aarch64 -M virt -cpu max,sve512=off
> +

I just noticed this while I was trying to understand the
SME and SVE property documentation -- the example 4 here
is in the SME property examples section, but it's changing
sve512, not sme512. Is that an error, or intentional? (If the
latter, the explanation could be enlarged upon to say that
it's changing behaviour of the SME vector length by adjusting
the SVE vector length.)

Also, the documentation for the SVE properties uses this
same command line, but it describes the effects differently:

# Disable the 512-bit vector length and all larger vector lengths,
# since 512 is a power-of-two. This results in all the smaller,
# uninitialized lengths (128, 256, and 384) defaulting to enabled:
#
# $ qemu-system-aarch64 -M virt -cpu max,sve512=off

In the SME section we say that all other lengths default
to enabled, but in the SVE section we say that the
smaller lengths default to enabled but the longer
lengths are disabled. Is:
 * the SVE part wrong?
 * the SME part wrong?
 * the behaviour deliberately different for SVE and SME
   vector lengths? (If so, we should say so explicitly to
   highlight that to users).

thanks
-- PMM