Re: [RFC 21/21] arm/cpu-features: Document custom vcpu model

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Oct 28, 2024 at 05:05:44PM +0100, Cornelia Huck wrote:
> On Fri, Oct 25 2024, Daniel P. Berrangé <berrange@redhat.com> wrote:
> 
> > On Fri, Oct 25, 2024 at 03:28:35PM +0200, Eric Auger wrote:
> >> Hi Daniel,
> >> 
> >> On 10/25/24 15:13, Daniel P. Berrangé wrote:
> >> > On Fri, Oct 25, 2024 at 12:17:40PM +0200, Eric Auger wrote:
> >> >> From: Cornelia Huck <cohuck@redhat.com>
> >> >>
> >> >> Add some documentation for the custom model.
> >> >>
> >> >> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> >> >> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> >> >> ---
> >> >>  docs/system/arm/cpu-features.rst | 55 +++++++++++++++++++++++++++-----
> >> >>  1 file changed, 47 insertions(+), 8 deletions(-)
> >> >
> >> >> @@ -167,6 +196,16 @@ disabling many SVE vector lengths would be quite verbose, the ``sve<N>`` CPU
> >> >>  properties have special semantics (see "SVE CPU Property Parsing
> >> >>  Semantics").
> >> >>  
> >> >> +The ``custom`` CPU model needs to be configured via individual ID register
> >> >> +field properties, for example::
> >> >> +
> >> >> +  $ qemu-system-aarch64 -M virt -cpu custom,SYSREG_ID_AA64ISAR0_EL1_DP=0x0
> >> >> +
> >> >> +This forces ID_AA64ISAR0_EL1 DP field to 0.
> >> > What is the "baseline" featureset implied by 'custom' ?
> >> there is no baseline at the moment. By default this is a host
> >> passthrough model.
> >
> > Why do we need to create "custom" at all, as opposed to just letting
> > users toggle features on "-cpu host" ? 
> 
> We could consolidate that to the current "host" model, once we figure
> out how to handle the currently already existing properties. Models
> based on the different architecture extensions would probably be more
> useable in the long run; maybe "custom" has a place for testing.

If you can set the features against "host", then any testing could
be done with "host" surely, making 'custom' pointless ?

With regards,
Daniel
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