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

[Michal Privoznik]

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.

Michal