Re: [Qemu-arm] [PATCH v2 07/14] target/arm/cpu64: max cpu: Introduce sve<vl-bits> properties

[Andrew Jones]

On Mon, Jun 24, 2019 at 12:05:26PM +0100, Dave Martin wrote:
> On Fri, Jun 21, 2019 at 05:34:15PM +0100, Andrew Jones wrote:
> > Introduce cpu properties to give fine control over SVE vector lengths.
> > We introduce a property for each valid length up to the current
> > maximum supported, which is 2048-bits. The properties are named, e.g.
> > sve128, sve256, sve512, ..., where the number is the number of bits.
> > 
> > It's now possible to do something like -cpu max,sve-max-vq=4,sve384=off
> > to provide a guest vector lengths 128, 256, and 512 bits. The resulting
> > set must conform to the architectural constraint of having all power-of-2
> > lengths smaller than the maximum length present. It's also possible to
> > only provide sve<vl-bits> properties, e.g. -cpu max,sve512=on. That
> > example provides the machine with 128, 256, and 512 bit vector lengths.
> > It doesn't hurt to explicitly ask for all expected vector lengths,
> > which is what, for example, libvirt should do.
> > 
> > Note1, it is not possible to use sve<vl-bits> properties before
> > sve-max-vq, e.g. -cpu max,sve384=off,sve-max-vq=4, as supporting
> > that overly complicates the user input validation.
> > 
> > Note2, while one might expect -cpu max,sve-max-vq=4,sve512=on to be the
> > same as -cpu max,sve512=on, they are not. The former enables all vector
> > lengths 512 bits and smaller, while the latter only sets the 512-bit
> > length and its smaller power-of-2 lengths. It's probably best not to use
> > sve-max-vq with sve<vl-bits> properties, but it can't be completely
> > forbidden as we want qmp_query_cpu_model_expansion to work with guests
> > launched with e.g. -cpu max,sve-max-vq=8 on their command line.
> 
> Supporting both options together in a non-idempotent way seems to
> complicate things.
> 
> Would it be simpler to allow sve-max-vq only when there are no sve<bits>
> options?

Not really. Since we can't simply remove sve-max-vq from the 'max' cpu
type, then we'd still need conditions to check when we can use it. The
restriction that it has to come first reduces the complexity
substantially, and I think restricting to not being allowed
at all, when sve<vl-bits> are used, would only give us a net check
reduction of one or two.

> 
> Alternatively, the two options would be defined so that their meanings
> are independent of parse order.
> 
> So, way sve-max-vq=<max> means that:
> 
>  * all VQs up to max for which there is no corresponding sve<bits>=off,
>    are enabled; and
> 
>  * VQ max is enabled; and
> 
>  * all VQs exceeding max are disabled.
> 
> While sve<bits>=(on|off) means
> 
>  * the VQ coresponding to <bits> is enabled (for on) or disabled (for
>    off).
>  
> After parsing all the options, you check that the sve-max-vq and
> sve<bits> optinos are consistent.  If you disallow duplicate sve-max-vq
> or sve<bits> options, then there is no possibility of ambiguity and the
> order or options doesn't matter.

I don't want to put any final checks at the end of parsing all options,
because that won't work with the QMP query.

> 
> (There may be constraints on the way qemu options parsing works that
> make this hard, though...)

I can't think of any issue with the parsing, but the QMP query only using
the property get accessors, without any finalizing, does put constraints
on what we can do.

> 
> Having sve-max-vq in 128-bit units while sve<bits> are named in terms of
> bit counts also feels a bit odd now.

sve-max-vq already exists, so it'd have to be deprecated if we don't want
it anymore. And I think sve<vl-bits> is the right naming to go with for
that. Of course using sve-max-vq is completely optional. If you don't want
it for backward compatible reasons, or as a shorthand to restrict the
lengths, then you can just use the sve<vl-bits> properties. Indeed, with
KVM, if you use the 'host' cpu type (the default for use with KVM), then
you won't even have the sve-max-vq property. As 'host' never had it, I
didn't have to keep it, nor adopt it. And of course I didn't want to
adopt it.

Thanks,
drew