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

From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Christoph Hellwig <hch@infradead.org>,
	<linux-kernel@vger.kernel.org>,
	Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>,
	David Airlie <airlied@linux.ie>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	"Juergen Gross" <jgross@suse.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	"Dan Carpenter" <dan.carpenter@oracle.com>, Wei Liu <wl@xen.org>,
	Yan Yankovskyi <yyankovskyi@gmail.com>,
	<dri-devel@lists.freedesktop.org>,
	<xen-devel@lists.xenproject.org>, <linux-mm@kvack.org>,
	David Hildenbrand <david@redhat.com>,
	Michal Hocko <mhocko@kernel.org>,
	Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH v4 2/2] xen: add helpers to allocate unpopulated memory
Date: Thu, 13 Aug 2020 12:02:26 +0200	[thread overview]
Message-ID: <20200813100226.GG975@Air-de-Roger> (raw)
In-Reply-To: <20200813094946.GJ2352366@phenom.ffwll.local>

Your email client seems to set 'Reply-to:' to point to everyone on the
'Cc:' field, but not yourself, which is kind of weird. I've manually
fixed it on this reply by moving everyone to the 'Cc:' field and
setting you on 'To:'.

On Thu, Aug 13, 2020 at 11:49:46AM +0200, Daniel Vetter wrote:
> On Thu, Aug 13, 2020 at 09:54:20AM +0200, Roger Pau Monné 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.
> 
> Why does arm not have ZONE_DEVICE?

It's not that Arm doesn't have ZONE_DEVICE, it's just that the
approach used here won't work correctly on an Arm Xen dom0 as-is.

This is due to the usage of an identity second stage translation in
order to workaround the lack of an IOMMU in some Arm boards.

It can be made to work on Arm, but will likely require someone from
the Arm side doing that.

Roger.