Re: [PATCH] docs/system/arm/virt.rst: Add note on CPU features off by default

[Peter Maydell]

On Mon, 22 Jan 2024 at 13:43, Gustavo Romero <gustavo.romero@linaro.org> wrote:
>
> Add a note on CPU features off by default on `virt` machines. These CPU
> features will remain off even if `-cpu max` is given so the user must
> turn on them explicitly on `virt` machines.
>
> Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>

Hi -- it looks like you forgot to cc qemu-devel on this;
could you do that on the next version, please? (All our
automated patch tooling looks at qemu-devel only. The
idea is that for other mailing lists you cc the other
list plus qemu-devel.)

> --- a/docs/system/arm/virt.rst
> +++ b/docs/system/arm/virt.rst
> @@ -69,6 +69,17 @@ Supported guest CPU types:
>  Note that the default is ``cortex-a15``, so for an AArch64 guest you must
>  specify a CPU type.
>
> +Also, please note that passing ``max`` CPU (i.e. ``-cpu max``) won't
> +enable all the CPU features for a given ``virt`` machine. Some CPU
> +features are ``off`` by default (e.g. MTE) -- see machine-specific list
> +below or check them for a given machine passing ``help``. For instance,
> +for the ``virt`` machine pass ``-machine virt,help`` to QEMU.
> +
> +For example, because MTE CPU feature is ``off`` by default it's
> +necessary to turn  MTE ``on`` explicitly to have this CPU feature
> +available on the ``virt`` machine, even when ``max`` CPU is specified:
> +``-machine virt,mte=on -cpu max`` .

I think I would phrase this as:

Where a CPU architectural feature requires support in
both the CPU itself and in the wider system, it may
not be enabled by default, but instead require a machine
option to enable it. For instance MTE support must be
enabled with ``-machine virt,mte=on``, as well as by
selecting an MTE-capable CPU with the ``-cpu`` option.
See the list of machine-specific options below.

> +
>  Graphics output is available, but unlike the x86 PC machine types
>  there is no default display device enabled: you should select one from
>  the Display devices section of "-device help". The recommended option

thanks
-- PMM