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Re: [RFC PATCH] xen/memory: Introduce a hypercall to provide unallocated space

Hi Julien, Stefano.

On 02.08.21 22:12, Oleksandr wrote:

Hi Stefano,

Thank you for the comments and ideas.

On 31.07.21 02:57, Stefano Stabellini wrote:
On Fri, 30 Jul 2021, Oleksandr wrote:
Hi Andrew, Julien.

@Andrew, I think that arguments you provided in your first answer (why the
proposed solution is a potentially racy and not a safe) are valid and
Thanks for the feedback.

On 28.07.21 22:53, Julien Grall wrote:

On 28/07/2021 20:00, Andrew Cooper wrote:
On 28/07/2021 18:27, Julien Grall wrote:
Hi Andrew,

On 28/07/2021 18:19, Andrew Cooper wrote:
On 28/07/2021 17:18, Oleksandr Tyshchenko wrote:
From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

Add XENMEM_get_unallocated_space hypercall which purpose is to
query hypervisor to find regions of guest physical address space
which are unused and can be used to create grant/foreign mappings
instead of wasting real pages from the domain memory for
establishing these mappings. The problem with the current Linux
on Xen on Arm behaviour is if we want to map some guest memory
regions in advance or to perform cache mappings in the backend
we might run out of memory in the host (see XSA-300).
This of course, depends on the both host and guest memory sizes.

The "unallocated space" can't be figured out precisely by
the domain on Arm without hypervisor involvement:
- not all device I/O regions are known by the time domain starts
     creating grant/foreign mappings
- the Dom0 is not aware of memory regions used for the identity
     mappings needed for the PV drivers to work
In both cases we might end up re-using these regions by
a mistake. So, the hypervisor which maintains the P2M for the domain
is in the best position to provide "unallocated space".
I'm afraid this does not improve the situation.

If a guest follows the advice from XENMEM_get_unallocated_space, and
subsequently a new IO or identity region appears, everything will
explode, because the "safe area" wasn't actually safe.

The safe range *must* be chosen by the toolstack, because nothing else
can do it safely or correctly.
The problem is how do you size it? In particular, a backend may map
multiple time the same page (for instance if the page is granted twice).
The number of mapped grants is limited by the size of the maptrack table
in Xen, which is a toolstack input to the domaincreate hypercall.
Therefore, the amount of space required is known and bounded.

There are a handful of other frames required in the current ABI (shared
info, vcpu info, etc).

The areas where things do become fuzzy is things like foreign mappings,
acquire_resource, etc for the control domain, which are effectively
unbounded from the domain's point of view.

For those, its entirely fine to say "here 128G of safe mapping space" or so.  Even the quantity of mapping dom0 can make is limited by the shadow memory pool and the number of pagetables Xen is willing to expend on the
second stage translation tables.
FWIW, on Arm, we don't have shadow memory pool.

Anyway, it should be possible to give a 128GB "safe range" and let Xen deal
with it.

Once a safe range (or ranges) has been chosen, any subsequent action
which overlaps with the ranges must be rejected, as it will violate
guarantees provided.

Furthermore, the ranges should be made available to the guest via
memory map means.  On x86, this is via the E820 table, and on ARM I
presume the DTB.  There is no need for a new hypercall.
Device-Tree only works if you have a guest using it. How about ACPI?
ACPI inherits E820 from x86 (its a trivial format), and UEFI was also
based on it.

But whichever...  All firmware interfaces have a memory map.
This will be UEFI memory map. However, I am a bit confused how we can tell the OS the region will be used for grant/foreign mapping. Is it possible to
reserved a new type?

To me the hypercall solution at least:
    1) Avoid to have to define the region on every single firmware table
There is only ever one.
Why? I could forsee an interest to use the host memory map and therefore we
may need to find a few holes for safe regions to use.

    2) Allow to easily extend after the guest run
The safe ranges can't be changed (safely).  This is the same problem as needing to know things like your PCI apertures ahead of time, or where
the DIMM hotplug regions are.

Having the guest physmap be actually dynamic is the cause of so many
bugs (inc security) and misfeatures in Xen.  Guests cannot and do no
cope with things being fully dynamic, because that's not how real
hardware works.  What you get is layers and layers of breakage on top of
each other, rather than a working system.
I would not call it "fully dynamic". Xen can easily know whether a region has ever be allocated before. So long the region has never been allocated,
then it should be fine. In fact...

The size of mapping space is a limit, just like maxphysaddr, or the PCI apertures, or MMCFG space, etc.  You can make it large by default (as it
doesn't consume resource when not being used), but any guest OS isn't
going to tolerate it morphing during runtime.
... I believe the OS may be not aware of the hotplug region until it is
actually used.

Anyway, I suggested this approach a few months ago to Oleksandr (see [1])
which BTW you were CCed on ;). My take was that Xen knows about the
unallocated space and therefore can make an informed decision without having
to allocate insanely large region.

Now if you think that's fine (IIRC Stefano had a preference for that as well). Then we can use the firmware table (assuming we can solve the ACPI

Well, if new hypercall and, what is more, "the querying hypervisor at runtime to find unused space" model itself is not welcome, I am ok, let's try to
create a working system,
may we please find a common ground to move this forward (at least on Arm for
now, the foreign mapping is the most important question).

I got the proposed idea in general, but I haven't connected all dots yet, some
points need clarification.

1. The safe range must be defined/allocated in advance and must remain const
during the runtime. The safe range must be chosen by the toolstack.
[For the initial implementation we can start with some large value (128GB) as
discussed above]


- Do we need to inform Xen about that range (via domain_create hypercall,
- What will be in charge of guaranteeing the safety of that range at runtime (reject new mapping requests with possible overlaps, etc), Xen, toolstack or
- Where that range should be located in guest address space, should that range
be the same for all domains (how GUEST_GNTTAB_BASE(SIZE) for example)
or it should be calculated based on actual guest_ram_base(size) for a
particular domain?
- What about a safe range the Dom0 can use itself? Xen should choose it for
Dom0 the same way how toolstack chooses it for other domains, correct?
Brainstorming an idea to answer some of these questions.

Xen needs to be able to choose a safe range without the toolstack's help
at least for dom0 and dom0less guests.

Completely agree.

Could we just have Xen choose the
safe range for all domains?

As I understand, the region info (base, size) should be known to whoever generates firmware tables (DTB) for the domain in order to insert that range. So, in case if Xen chooses that range, it should be queried by the toolstack anyway (either by new domctl or extending existing one which context would fit).
This adds some complexity.

Could it be just as simple as choosing a range 1GB-aligned above
whatever is the highest address previously allocated for memory, MMIO,
Probably, we could choose a range above max mapped gpfn at the domain creation time. Currently, we are dealing with platform devices only on Arm and all IO are known in advance, so no new IO are expected at runtime. But, what to do when hotplug comes into play (and new conflicting IO might just appear)? On the other side, we might be faced with the same problem, if we used a predefined static range. With both options (calculated and predefined range) we would need to deny any new conflicting mapping.

We could also have the toolstack provide the info but I wonder if we can
get away without it.

I may not see all pitfalls here, but it seems that having the same predefined range for all domains would simplify things.
On Arm we have the following:

#define GUEST_RAM1_BASE   xen_mk_ullong(0x0200000000) /* 1016GB of RAM @ 8GB */
#define GUEST_RAM1_SIZE   xen_mk_ullong(0xfe00000000)

Now I am wondering, can we limit the GUEST_RAM1_SIZE by 128GB for example in order to reserve a range at the end, would this be acceptable? So, both Xen and toolstack would see that definition and would be able to insert a range to the generated DTB, Xen's P2M code would be in change of keeping that range in a safe state.

2. The safe range must be provided to domain using the firmware table.
[We can start with the DTB and leave ACPI unimplemented for now, assuming we
will be able to solve open questions as discussed above]


- Do we need distinguish between foreign and grant mappings at the domain side
at all? Can the same safe range be used for all types of mapping?
This is a good question. I can't think of a reason why we would need to
distinguish between the two from Xen's point of view. Would it make
the Linux implementation easier if we distinguished them?
I may mistake, but I don't think it would make the Linux implementation easier, both foreign and grants mappings result in same action which is xen_alloc_unpopulated_pages (or alloc_xenballooned_pages). For both mappings the problem with wasting real domain memory on Arm is actual at the moment, although with foreign mapping
for the device emulator use case it becomes more evident.

- How will domain recognize what range can be used for foreign/grant mappings? Do we need to introduce new device-tree bindings for describing the range or it is possible to re-use current bindings (expand the "reg" property under
"hypervisor" node, currently it contains gnttab region)?
Do we need a struct page* for the range? We needed it in the past and it
is the reason why we currenly use ballooned-out pages: they have a
proper struct page* associated with them. pte_special() caused problems.
So, continuing with the assumption that we need a struct page*, then the
range needs to be advertised as "memory" to the DomU (part to of the
/memory node). But we can reserve the range for foreign/grant-mapping
use by adding a reserved-memory binding for it. If we go down this
route, we should add a new binding as I don't think we should reuse
reserved-memory/xen,shared-memory.txt. It would be very simply to add,
just follow the model of xen,shared-memory.txt. (Note that just because
the range is part of the DomU /memory node, it doesn't mean it needs to
be backed by real memory.)

If we don't need a struct page* then we can do something different using
bus ranges and/or MMIO regions.

Yes, we need struct page* for the each pfn in the range. But, I think, this can be achieved even without describing the range as a part of memory/reserved-memory nodes. Fortunately, the Linux ZONE_DEVICE facility can help with that, actually the "unpopulated-alloc" is based on this feature, please see drivers/xen/unpopulated-alloc.c:fill_list() and https://lore.kernel.org/lkml/1627490656-1267-2-git-send-email-olekstysh@xxxxxxxxx/ for details.

I have done some experiments with Xen and toolstack according to the discussion above. So, I re-used DTB to pass a safe range to the domain. For the range I borrowed some space from the second RAM bank.

-#define GUEST_RAM1_BASE   xen_mk_ullong(0x0200000000) /* 1016GB of RAM @ 8GB */
-#define GUEST_RAM1_SIZE   xen_mk_ullong(0xfe00000000)
+#define GUEST_RAM1_BASE   xen_mk_ullong(0x0200000000) /* 888GB of RAM @ 8GB */
+#define GUEST_RAM1_SIZE   xen_mk_ullong(0xDE00000000)
+#define GUEST_SAFE_RANGE_BASE   xen_mk_ullong(0xDE00000000) /* 128GB */
+#define GUEST_SAFE_RANGE_SIZE   xen_mk_ullong(0x0200000000)

While the possible new DT bindings has not been agreed yet, I re-used existing "reg" property under the hypervisor node to pass safe range as a second region,

--- a/tools/libs/light/libxl_arm.c
+++ b/tools/libs/light/libxl_arm.c
@@ -735,9 +735,11 @@ static int make_hypervisor_node(libxl__gc *gc, void *fdt,
     if (res) return res;

-    /* reg 0 is grant table space */milat
+    /* reg 0 is grant table space, reg 1 is safe range */
     res = fdt_property_regs(gc, fdt, GUEST_ROOT_ADDRESS_CELLS, GUEST_ROOT_SIZE_CELLS,
-                            1,GUEST_GNTTAB_BASE, GUEST_GNTTAB_SIZE);
+                            2,
+                            GUEST_GNTTAB_BASE, GUEST_GNTTAB_SIZE,
+                            GUEST_SAFE_RANGE_BASE, GUEST_SAFE_RANGE_SIZE);
     if (res) return res;


/* Resulting hypervisor node */

 hypervisor {
                interrupts = <0x01 0x0f 0xf08>;
                interrupt-parent = <0xfde8>;
                compatible = "xen,xen-4.16\0xen,xen";
                reg = <0x00 0x38000000 0x00 0x1000000 0xde 0x00 0x02 0x00>;

Near the same I did for the Xen itself to insert a range for Dom0. The Linux side change is just to retrieve a range from DTB instead of issuing a hypercall.

Sorry, I might miss some important bits here, but from what I wrote about the "reg" purpose, it seems it could be suitable for us, why actually not? Why do we need yet another binding? I noticed, Linux on Arm doesn't use it at all, probably it is used by other OSes, I don't know.

Now, I am wondering, would it be possible to update/clarify the current "reg" purpose and use it to pass a safe unallocated space for any Xen specific mappings (grant, foreign, whatever) instead of just for the grant table region. In case, it is not allowed for any reason (compatibility PoV, etc), would it be possible to extend a property by passing an extra range separately, something similar to how I described above?


Oleksandr Tyshchenko



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