Re: [PATCH v3] machine: add missing doc for memory-backend option

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Jan 27, 2021 at 03:24:26PM +0100, Michal Privoznik wrote:
> On 1/27/21 11:54 AM, Daniel P. Berrangé wrote:
> > On Wed, Jan 27, 2021 at 10:45:11AM +0000, Daniel P. Berrangé wrote:
> > > On Thu, Jan 21, 2021 at 11:15:04AM -0500, Igor Mammedov wrote:
> 
> 
> > > 
> > > How does a mgmt app know which machine types need to use this
> > > option ? The machine type names are opaque strings, and apps
> > > must not attempt to parse or interpret the version number
> > > inside the machine type name, as they can be changed by
> > > distros.  IOW, saying to use it for machine types 4.0 and
> > > older isn't a valid usage strategy IMHO.
> > 
> > Looking at the libvirt patch, we do indeed use his property
> > unconditionally for all machine types, precisely because parsing
> > version numbers from the machine type is not allowed.
> > 
> > https://www.redhat.com/archives/libvir-list/2021-January/msg00633.html
> > 
> > So this doc is telling apps to do something that isn't viable
> 
> The other approach that I was suggesting was, that QEMU stops reporting
> 'default-ram-id' for affected machine types. The way the switch from '-m
> XMB' to memory-backend-* was implemented in libvirt is that if libvirt sees
> 'default-ram-id' attribute for given machine type it uses memory-backend-*
> otherwise it falls back to -m.
> 
> Since we know which machine types are "broken", we can stop reporting the
> attribute and thus stop tickling this bug. I agree that it puts more burden
> on distro maintainers to backport the change, but I think it's acceptable
> risk.

IIUC That's only a burden for distros if they're creating their own
machine types, in which case they've already decided the burden is
a net win.


Regards,
Daniel
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