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

[Dave Martin]

On Wed, May 15, 2019 at 12:09:04PM +0100, Dr. David Alan Gilbert wrote:
> * Dave Martin (address@hidden) wrote:
> > On Wed, May 15, 2019 at 09:18:54AM +0100, Andrew Jones wrote:
> > > On Tue, May 14, 2019 at 04:48:38PM +0200, Igor Mammedov wrote:
> > > > On Tue, 14 May 2019 11:02:25 +0200
> > > > Andrew Jones <address@hidden> wrote:
> > > > > My thought is primarily machines. If a human wants to use the command
> > > > > line and SVE, then I'm assuming they'll be happy with sve-max-vq or
> > > > > figuring out a map they like once and then sticking to it.
> > > > 
> > > > maybe naive question, but why not use a property/bit as user facing 
> > > > interface,
> > > > in line with what we do with CPUID bits. (that's assuming that bits have
> > > > fixed meaning).
> > > > Yes, it's verbose but follows current practice and works fine with -cpu 
> > > > and
> > > > -device.
> > > > (I really hate custom preprocessing of -cpu and we were working hard to 
> > > > remove
> > > > that in favor of canonical properties at the expense of more verbose 
> > > > CLI).
> > > >
> > > 
> > > Are you asking if we should do something like the following?
> > > 
> > >   -cpu host,sve1=on,sve=2=on,sve3=off,sve4=on
> > 
> > Note, there is nothing SVE-specific about this.
> > 
> > Either enabling features on a per-vcpu basis is justified, or it isn't:
> > if it's justified, then it would be better to have a general way of
> > specifying per-vcpu properties, rather than it being reinvented per
> > feature.
> 
> SVE *is* a bit unusual.  In most CPU features they're actually features,
> they're on or off, so we have a big list of features that are
> enabled/disabled.  We've had that type of thing (at least on x86) for
> years and it's OK.
> We've got one or two things where they're numerical
> (e.g. host-physbits) and we struggle a bit with how to handle them.
> 
> SVE is somewhere in between - it's a list of numbers, apparently a
> fairly large arbitrarily set of numbers that could be chosen so you'd
> need lots of feature flags (sve1...sve64 say or more); so that doesn't
> fit the existing things we've had that have worked.

I see what you mean now.  SVE configuration is indeed more than just a
boolean flag.

Cheers
---Dave