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

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

On Fri, Aug 14, 2020 at 02:54:38PM +0200, Jürgen Groß wrote:
> On 14.08.20 14:47, Roger Pau Monné wrote:
> > 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.
> 
> I just want to avoid that kernels built for running as Xen guest, but
> not as dom0, will be forced to select ZONE_DEVICE.
> 
> As privcmd is dom0-only, this is no problem.

Oh, didn't know that, I somehow assumed privcmd would be available to
all Xen guests regardless of whether dom0 support was selected.

> In case you are worrying about gnt devices, I'd be fine to switch to
> 
> default XEN_BACKEND || XEN_GNTDEV

Sure. maybe even:

default XEN_BACKEND || XEN_GNTDEV || XEN_DOM0

Do you want me to resend with this changed or are you happy to fixup
if there are no further comments?

Thanks, Roger.