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

From: Michal Privoznik
Subject: Re: [PATCH v3] machine: add missing doc for memory-backend option
Date: Wed, 27 Jan 2021 19:30:24 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.1

On 1/27/21 6:56 PM, Daniel P. Berrangé wrote:
On Wed, Jan 27, 2021 at 04:35:22PM +0100, Igor Mammedov wrote:
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).

IIUC, this means that setting the property has no impact on
migration ABI for new machine types > 4.0....

Limiting knob usage to 4.0 and older would allow us to drop
without extra efforts once 4.0 is deprecated/removed.

...so, even if we set the property unconditionally for *all*
machine types, then we can still remove it in future, becuase
its removal won't affect ABI of the 5.x, 6.x machine types.

Alright, so after all you agree with proposed patch? I'm a bit confused.

At any rate, we have patches for nearly all available options (if not all), and I'd like to get things moving because both qemu and libvirt were already released meanwhile and our upstreams are broken. Not to mention users that started domain with libvirt 7.0.0 - they will be unable to migrate to libvirt 7.1.0+ (or whatever version we fix this), because libvirt 7.0.0 did not set the x-something-something attribute.


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