Re: [PATCH v4 2/2] xen: add helpers to allocate unpopulated memory

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Aug 14, 2020 at 12:27:32PM +0200, Jürgen Groß wrote:
> On 14.08.20 11:56, Roger Pau Monné wrote:
> > On Fri, Aug 14, 2020 at 08:29:20AM +0100, Christoph Hellwig wrote:
> > > On Thu, Aug 13, 2020 at 09:54:20AM +0200, Roger Pau Monn?? wrote:
> > > > On Thu, Aug 13, 2020 at 08:33:37AM +0100, Christoph Hellwig wrote:
> > > > > On Tue, Aug 11, 2020 at 11:44:47AM +0200, Roger Pau Monne wrote:
> > > > > > If enabled (because ZONE_DEVICE is supported) the usage of the new
> > > > > > functionality untangles Xen balloon and RAM hotplug from the usage 
> > > > > > of
> > > > > > unpopulated physical memory ranges to map foreign pages, which is 
> > > > > > the
> > > > > > correct thing to do in order to avoid mappings of foreign pages 
> > > > > > depend
> > > > > > on memory hotplug.
> > > > > 
> > > > > So please just select ZONE_DEVICE if this is so much better rather
> > > > > than maintaining two variants.
> > > > 
> > > > We still need to other variant for Arm at least, so both need to be
> > > > maintained anyway, even if we force ZONE_DEVICE on x86.
> > > 
> > > Well, it still really helps reproducability if you stick to one
> > > implementation of x86.
> > > 
> > > The alternative would be an explicit config option to opt into it,
> > > but just getting a different implementation based on a random
> > > kernel option is strange.
> > 
> > Would adding something like the chunk below to the patch be OK?
> > 
> > ---8<---
> > diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> > index 018020b91baa..5f321a1319e6 100644
> > --- a/drivers/xen/Kconfig
> > +++ b/drivers/xen/Kconfig
> > @@ -328,7 +328,14 @@ config XEN_FRONT_PGDIR_SHBUF
> >     tristate
> >   config XEN_UNPOPULATED_ALLOC
> > -   bool
> > -   default y if ZONE_DEVICE && !ARM && !ARM64
> > +   bool "Use unpopulated memory ranges for guest mappings"
> > +   depends on X86
> > +   select ZONE_DEVICE
> > +   default y
> 
> I'd rather use "default XEN_BACKEND" here, as mappings of other guest's
> memory is rarely used for non-backend guests.

There's also the privcmd and gnt devices which make heavy use of this,
so I'm not sure only selecting by default on XEN_BACKEND is the best
option.

Thanks, Roger.