[v2] docs/system/arm/virt.rst: Add note on CPU features off by default

Commit Message

Gustavo Romero Jan. 22, 2024, 9:12 p.m. UTC

Add a note on CPU features that are off by default in `virt` machines.
Some CPU features will remain off even if a CPU-capable CPU (e.g.,
`-cpu max`) is selected because they require support in both the CPU
itself and in the wider system. Therefore, the user, besides selecting a
CPU that supports such features, must also turn on the feature using a
machine option.

Signed-off-by: Gustavo Romero <[e-mail]>
---
 docs/system/arm/virt.rst | 13 +++++++++++++
 1 file changed, 13 insertions(+)

Comments

Peter Maydell Jan. 25, 2024, 12:49 p.m. UTC | #1

On Mon, 22 Jan 2024 at 21:12, Gustavo Romero <[e-mail]> wrote:
>
> Add a note on CPU features that are off by default in `virt` machines.
> Some CPU features will remain off even if a CPU-capable CPU (e.g.,
> `-cpu max`) is selected because they require support in both the CPU
> itself and in the wider system. Therefore, the user, besides selecting a
> CPU that supports such features, must also turn on the feature using a
> machine option.
>
> Signed-off-by: Gustavo Romero <[e-mail]>

Applied to target-arm.next, thanks.

-- PMM

Patch

diff --git a/docs/system/arm/virt.rst b/docs/system/arm/virt.rst
index c245c52b7a..1888e31956 100644
--- a/docs/system/arm/virt.rst
+++ b/docs/system/arm/virt.rst
@@ -69,6 +69,19 @@  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. Where a CPU
+architectural feature requires support in both the CPU itself and in the
+wider system (e.g. the MTE feature), it may not be enabled by default,
+but instead requires a machine option to enable it.
+
+For example, MTE support must be enabled with ``-machine virt,mte=on``,
+as well as by selecting an MTE-capable CPU (e.g., ``max``) with the
+``-cpu`` option.
+
+See the machine-specific options below or check them for a given machine
+passing ``help``, like: ``-machine virt-9.0,help``.
+
 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