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

[Dave Martin]

On Thu, Apr 18, 2019 at 03:43:06PM +0100, Andrew Jones wrote:
> On Thu, Apr 18, 2019 at 03:03:02PM +0100, Dave Martin wrote:

[...]

> > It's worth nothing that there are two problems to be solved here: one is
> > to specify an exact set unambiguously, which is important for migration
> > scenarios.
> > 
> > The other is to be able to clamp the vector length for user convenience,
> > but without particularly considering migration.  There's no direct way
> > for the user to know the set of vector lengths supported by KVM today,
> > so cooking up the right magic to go on the command line is non-trivial:
> > you have to create a dummy vcpu and read KVM_REG_ARM64_SVE_VLS to find
> > the host's supported set.  Or you just have to know.
> 
> Right, we'll have to query first to know what's available. libvirt will
> do that and tools built on libvirt should help provide the user with
> sane defaults or a relatively painless selection process. The QEMU command
> line is primarily for developers, so they'll likely just know what they
> want.
> 
> > 
> > A separate max-vl option (or whatever) might be a useful alternative.
> 
> Yeah, we could offer this to be nicer to QEMU command line users that
> don't intend to migrate, or don't care if a migration can fail. This
> would be analogous to '-cpu max' which enables all available cpu
> features. Bad for migration, but a nice shorthand if you want your
> guest to have everything.

Sounds reasonable.

Cheers
---Dave