Re: [PATCH v3 4/4] xen: add helpers to allocate unpopulated memory

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

On Tue, Jul 28, 2020 at 06:12:46PM +0100, Julien Grall wrote:
> Hi Roger,
> 
> On 28/07/2020 17:59, Roger Pau Monné wrote:
> > On Tue, Jul 28, 2020 at 05:48:23PM +0100, Julien Grall wrote:
> > > Hi,
> > > 
> > > On 27/07/2020 10:13, Roger Pau Monne wrote:
> > > > To be used in order to create foreign mappings. This is based on the
> > > > ZONE_DEVICE facility which is used by persistent memory devices in
> > > > order to create struct pages and kernel virtual mappings for the IOMEM
> > > > areas of such devices. Note that on kernels without support for
> > > > ZONE_DEVICE Xen will fallback to use ballooned pages in order to
> > > > create foreign mappings.
> > > > 
> > > > The newly added helpers use the same parameters as the existing
> > > > {alloc/free}_xenballooned_pages functions, which allows for in-place
> > > > replacement of the callers. Once a memory region has been added to be
> > > > used as scratch mapping space it will no longer be released, and pages
> > > > returned are kept in a linked list. This allows to have a buffer of
> > > > pages and prevents resorting to frequent additions and removals of
> > > > regions.
> > > > 
> > > > 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.
> > > I think this is going to break Dom0 on Arm if the kernel has been built 
> > > with
> > > hotplug. This is because you may end up to re-use region that will be used
> > > for the 1:1 mapping of a foreign map.
> > > 
> > > Note that I don't know whether hotplug has been tested on Xen on Arm yet. 
> > > So
> > > it might be possible to be already broken.
> > > 
> > > Meanwhile, my suggestion would be to make the use of hotplug in the 
> > > balloon
> > > code conditional (maybe using CONFIG_ARM64 and CONFIG_ARM)?
> > 
> > Right, this feature (allocation of unpopulated memory separated from
> > the balloon driver) is currently gated on CONFIG_ZONE_DEVICE, which I
> > think could be used on Arm.
> > 
> > IMO the right solution seems to be to subtract the physical memory
> > regions that can be used for the identity mappings of foreign pages
> > (all RAM on the system AFAICT) from iomem_resource, as that would make
> > this and the memory hotplug done in the balloon driver safe?
> 
> Dom0 doesn't know the regions used for the identity mappings as this is only
> managed by Xen. So there is nothing you can really do here.

OK, I will add the guards to prevent this being built on Arm.

> But don't you have the same issue on x86 with "magic pages"?

Those are marked as reserved on the memory map, and hence I would
expect them to never end up in iomem_resource.

Thanks, Roger.