Re: [PATCH v7 08/12] multifd: Add capability to enable/disable zero_page

[Juan Quintela]

Leonardo Brás <leobras@redhat.com> wrote:
> On Tue, 2022-08-02 at 08:39 +0200, Juan Quintela wrote:
>> We have to enable it by default until we introduce the new code.
>> 
>> Signed-off-by: Juan Quintela <quintela@redhat.com>
>> 
>> ---
>> 
>> Change it to a capability.  As capabilities are off by default, have
>> to change MULTIFD_ZERO_PAGE to MAIN_ZERO_PAGE, so it is false for
>> default, and true for older versions.
>
> IIUC, the idea of a capability is to introduce some new features to the code,
> and let users enable or disable it. 

All capabilities are false by default.
If we change the capability to be true by default, we need to teach
libvirt new tricks.

> If it introduce a new capability, is not very intuitive to think that it will 
> be
> always true for older versions, and false for new ones.

It don't need to be intuitive, it just need to be documented correctly.
I think that is done, no?

> I would suggest adding it as MULTIFD_ZERO_PAGE, and let it disabled for now.
> When the full feature gets introduced, the capability could be enabled by
> default, if desired.

I have it that way before it was a capability.
but the info migrate_capabilities

showed everything false and this one true, wondering _why_ this one is
true.

So I decided to rename it, and make it true by default.

> What do you think?

I preffer it this way, why?

Because at some point in the future, we will remove the code that
implements the capability and the capability.  So the idea is that we
don't want to use it for old code.

Later, Juan.