Re: [PATCH v4 4/9] target/arm/cpu64: max cpu: Introduce sve<N> properties

[Andrew Jones]

On Tue, Sep 24, 2019 at 01:31:00PM +0200, Andrew Jones wrote:
> +SVE CPU Property Parsing Semantics
> +----------------------------------
> +
> +  1) If SVE is disabled (`sve=off`), then which SVE vector lengths
> +     are enabled or disabled is irrelevant to the guest, as the entire
> +     SVE feature is disabled and that disables all vector lengths for
> +     the guest.  However QEMU will still track any `sve<N>` CPU
> +     properties provided by the user.  If later an `sve=on` is provided,
> +     then the guest will get only the enabled lengths.

I just spoke with Andrea, who is doing the libvirt side of this, about
this behavior. He suggests that we change it such that an error is
generated if sve=off and there are explicitly enabled vector lengths,
e.g. '-cpu max,sve=off,sve256=on' should generate an error. It would
still be possible to have sve<N>=on and sve=off on the same command
line, as long as the sve<N>=on is followed by sve<N>=off, like this
-cpu max,sve=off,sve256=on,sve256=off,sve128=off, but I don't think
we need to worry about that sort of thing. I'll add an error for
the non-empty vector length map with SVE off case for v5.

Thanks,
drew