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

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

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
+       help
+         Use unpopulated memory ranges in order to create mappings for guest
+         memory regions, including grants maps and foreign pages. This avoids
+         having to balloon out RAM regions in order to obtain physical memory
+         space to create such mappings.
 
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