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

Hi Oleksandr,

On 05/08/2021 15:52, Oleksandr wrote:

On 05.08.21 01:00, Julien Grall wrote:

On 04/08/2021 21:56, Oleksandr wrote:

Hi Julien, Stefano.

Hi Oleksandr,

Hi, Julien

Thank you for the prompt reply and explanations.

On 02.08.21 22:12, Oleksandr wrote:
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)

I am a bit split with reducing the amount of RAM. On one hand large guest is not unheard on the server side (at least in the x86 world). On the other hand, I am not aware of anyone using Xen on Arm in such setup.

So technically this will be a regression, but it may be OK.

I got it.

Regarding the range, this will be a problem as Xen configure the number of the IPA bits based on the PA bits. The lowest possible address space ize on 64-bit is 4GB.

From my understanding, this is because the number of IPA bits supported is contrained by the PA bits. So the position and the size of the region
would need to depend on the P2M configuration.

Indeed, I missed these bits that IPA bits on Arm64 might be < 40 bit, I remember, we select p2m_ipa_bits in setup_virt_paging() depending on pabits, moreover the p2m_ipa_bits might be even restricted by some external entity (IOMMU, if P2M is shared).

For simplicity, this could be the last few X bytes of the supported address space.
ok, agree. To summarize, so it sounds like we can't use the fixed safe range as in my example, it must be variable. Well, I hope, we will be able to achieve this without reducing the total amount of domain RAM in front (GUEST_RAM1_SIZE). After all, we know the IPA size and the domain RAM in advance, so we certainly can choose the start and size of the range. In case, we won't be able to find a suitable large chunk (for example, when IPA bits = 32, and domain has a lot of RAM assigned and as the result - almost all address space below 4GB is in use), we won't expose a safe range to a domain at all, and domain will just fall back to use real pages instead (actually, how it currently behaves on Arm).

I think it would be fine for a first approach. We can refine it in the future. What matters is that we correctly define the binding/hypercall.

A side note: we would likely need the toolstack (that generates device-tree for guests) to query IPA size, or similar.

I think we can use XEN_SYSCTL_* (or possibly hypfs) for that.

For 32-bit domain, we also need to make sure the address is usable for domain short page tables (not too long ago Debian was shipping the kernel with them rather than LPAE). I haven't yet checked what's the limit here.

Hmm, I didn't take this use-case into the account. So, I assume we need the safe range to be located below 4GB if is_32bit_domain() returns true.

Yes. Or we can say that if you are using a 32-bit domain then we don't (yet) support a safe range for range.
So we would need some heuristic to decide whether to stole some RAM or use the safe space. Another possibility would be to add a new compatible in the DT that indicates the region is "big" enough.
I like the last idea, did you perhaps mean new property (optional) rather than new compatible? Let's say "xen, safe-range" or "xen, extended-regions"  ...

I actually meant adding an extra compatible because this is technically a change of the binding (even though it is backward compatible).

Although, I would be OK with the property. You may first want to ask the Device-Tree folks how they expect a binding to be extended in a backward compatible way.


Julien Grall



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