Re: [PATCH V1 17/26] machine: memfd-alloc option

[Peter Xu <peterx@redhat.com>]

On Tue, Jun 04, 2024 at 08:13:26AM +0100, Daniel P. Berrangé wrote:
> On Mon, Jun 03, 2024 at 05:48:32PM -0400, Peter Xu wrote:
> > That property, irrelevant of what it is called (and I doubt whether Dan's
> > suggestion on "shared-ram" is good, e.g. mmap(MAP_SHARED) doesn't have user
> > visible fd but it's shared-ram for sure..), is yet another way to specify
> > guest mem types.
> > 
> > What if the user specified this property but specified something else in
> > the -object parameters?  E.g. -machine share-ram=on -object
> > memory-backend-ram,share=off.  What should we do?
> 
> The machine property would only apply to memory regions that are
> *NOT* being created via -object. The memory-backend objects would
> always honour their own share settnig.

In that case we may want to rename that to share-ram-by-default=on.
Otherwise it's not clear which one would take effect from an user POV, even
if we can define it like that in the code.

Even with share-ram-by-default=on, it can be still confusing in some form
or another. Consider this cmdline:

  -machine q35,share-ram-by-default=on -object memory-backend-ram,id=mem1

Then is mem1 shared or not?  From reading the cmdline, if share ram by
default it should be ON if we don't specify it, but it's actually off?
It's because -object has its own default values.

IMHO fundamentally it's just controversial to have two ways to configure
guest memory.  If '-object' is the preferred and complete way to configure
it, I prefer sticking with it if possible and see what is missing.

I think I raised that as the other major reason too, that I think it's so
far only about the vram that is out of the picture here.  We don't and
shouldn't have complicated RW RAMs floating around that we want this
property to cover.  We should make sure we introduce this property with
some good reason, rather than "ok we don't know what happened, we don't
know what will leverage this, but maybe there is some floating RAMs..".
Right after we introduce this property we need to carry it forever, and
prepared people start to use it with/without cpr, and that's some
maintenance cost that I want to see whether we can avoid.

Thanks,

-- 
Peter Xu