Re: [Qemu-devel] How do we do user input bitmap properties?

[Andrea Bolognani]

On Wed, 2019-05-15 at 13:54 +0200, Andrew Jones wrote:
> On Wed, May 15, 2019 at 12:52:29PM +0200, Igor Mammedov wrote:
> > since using magic numbers is not very descriptive
> > (but if there is some spec where they come from that we could point users to
> > it might be acceptable too, but I'd reserve number approach for values 
> > only).
> 
> The numbers aren't magic, they're part of the name. '1' in the above
> 'sve1' means one quadword. It would probably have been better to use bits
> instead in the example, i.e.
> 
>   -cpu host,sve128=on,sve256=on,sve384=off,sve512=on
> 
> where it's now clear that "sve512" has an analogy with x86's "avx512".
> 
[...]
> 
> So I set off to convince Igor of the wide word idea (he sits next to me,
> so I didn't have go far), but he has convinced me of the above property
> idea. He used the magic phrase: "less code would be needed". If we use
> the properties like above then we get introspection for free (cpu property
> listing which libvirt already knows how to do) - so no QMP query needed.
> The cost is adding several properties (16 to handle the current 2048-bit
> limit), but I guess that's cheap enough. The command line is verbose, but
> also much easier for a human to construct and read. I'm pretty sold on
> this path, but adding Andrea and Eduardo for their input as well.

Sorry for taking a while to respond. Anyway, the above looks good to
me as a general direction, but note that you'll have to implement at
the very least the query-cpu-model-expansion QMP command for the
introspection to work.

query-cpu-model-baseline and query-cpu-model-comparison are two more
QMP command which, while perhaps not immediately applicabile, we will
want to implement at some point; more in general, what s390x is doing
with respect to CPU models is a good blueprint, according to the
libvirt developer who's the most involved with that specific area of
the project.

-- 
Andrea Bolognani / Red Hat / Virtualization