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

From: "Roger Pau Monné" <roger.pau@citrix.com>
To: "Jürgen Groß" <jgross@suse.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	<linux-kernel@vger.kernel.org>,
	Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>,
	David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>,
	Boris Ostrovsky <boris.ostrovsky@oracle.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: Fri, 14 Aug 2020 15:35:36 +0200	[thread overview]
Message-ID: <20200814133536.GL975@Air-de-Roger> (raw)
In-Reply-To: <fc10d3a5-fe79-335e-353a-1bf480af7c7c@suse.com>

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.