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

From: Igor Mammedov
Subject: Re: [PATCH v3] machine: add missing doc for memory-backend option
Date: Wed, 27 Jan 2021 16:35:22 +0100

On Wed, 27 Jan 2021 15:24:26 +0100
Michal Privoznik <mprivozn@redhat.com> 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.
it's possible (but no necessary) to use knob with new machine types
(defaults for these match suggested property value).
Limiting knob usage to 4.0 and older would allow us to drop
without extra efforts once 4.0 is deprecated/removed.

> > 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.

default-ram-id is already exposed in wild including old machine types
starting from 5.2

if libvirt will take care this one quirk, then I guess we can
do as suggested. I can post an additional patch to this effect if there
is agreement to go this route.

my take on it, at this point is that it's not worth the effort,
we can just use option unconditionally and flow usual deprecate/remove
process once 4.0 machine type is removed.

> Michal

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