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

From: Eduardo Habkost
Subject: Re: [Qemu-devel] How do we do user input bitmap properties?
Date: Fri, 24 May 2019 15:24:34 -0300
User-agent: Mutt/1.10.1 (2018-07-13)

On Thu, May 23, 2019 at 10:35:24AM +0200, Andrea Bolognani wrote:
> 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.

Why is query-cpu-model-expansion needed?  Isn't
device-list-properties enough?

> 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.

Agreed.  Even if not necessary right now, query-cpu-model-* will
probably be needed eventually.


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