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Re: Virtio in Xen on Arm (based on IOREQ concept)

Hi Roger,

On 22/07/2020 09:21, Roger Pau Monné wrote:
On Tue, Jul 21, 2020 at 10:12:40PM +0100, Julien Grall wrote:
Hi Oleksandr,

On 21/07/2020 19:16, Oleksandr wrote:

On 21.07.20 17:27, Julien Grall wrote:
On a similar topic, I am a bit surprised you didn't encounter memory
exhaustion when trying to use virtio. Because on how Linux currently
works (see XSA-300), the backend domain as to have a least as much
RAM as the domain it serves. For instance, you have serve two
domains with 1GB of RAM each, then your backend would need at least
2GB + some for its own purpose.

This probably wants to be resolved by allowing foreign mapping to be
"paging" out as you would for memory assigned to a userspace.

Didn't notice the last sentence initially. Could you please explain your
idea in detail if possible. Does it mean if implemented it would be
feasible to map all guest memory regardless of how much memory the guest

Avoiding map/unmap memory each guest request would allow us to have
better performance (of course with taking care of the fact that guest
memory layout could be changed)...

I will explain that below. Before let me comment on KVM first.

Actually what I understand looking at kvmtool is the fact it does not
map/unmap memory dynamically, just calculate virt addresses according to
the gfn provided.

The memory management between KVM and Xen is quite different. In the case of
KVM, the guest RAM is effectively memory from the userspace (allocated via
mmap) and then shared with the guest.

 From the userspace PoV, the guest memory will always be accessible from the
same virtual region. However, behind the scene, the pages may not always
reside in memory. They are basically managed the same way as "normal"
userspace memory.

In the case of Xen, we are basically stealing a guest physical page
allocated via kmalloc() and provide no facilities for Linux to reclaim the
page if it needs to do it before the userspace decide to unmap the foreign

I think it would be good to handle the foreing mapping the same way as
userspace memory. By that I mean, that Linux could reclaim the physical page
used by the foreing mapping if it needs to.

The process for reclaiming the page would look like:
     1) Unmap the foreign page
     2) Ballon in the backend domain physical address used by the foreing
mapping (allocate the page in the physmap)

The next time the userspace is trying to access the foreign page, Linux will
receive a data abort that would result to:
     1) Allocate a backend domain physical page
     2) Balloon out the physical address (remove the page from the physmap)
     3) Map the foreing mapping at the new guest physical address
     4) Map the guest physical page in the userspace address space

This is going to shatter all the super pages in the stage-2

Yes, but this is nothing really new as ballooning would result to (AFAICT) the same behavior on Linux.

With this approach, we should be able to have backend domain that can handle
frontend domain without require a lot of memory.

Linux on x86 has the option to use empty hotplug memory ranges to map
foreign memory: the balloon driver hotplugs an unpopulated physical
memory range that's not made available to the OS free memory allocator
and it's just used as scratch space to map foreign memory. Not sure
whether Arm has something similar, or if it could be implemented.

We already discussed that last year :). This was attempted in the past (I was still at Citrix) and indefinitely paused for Arm.

/proc/iomem can be incomplete on Linux if we didn't load a driver for all the devices. This means that Linux doesn't have the full view of what is physical range is freed.

Additionally, in the case of Dom0, all the regions corresponding to the host RAM are unusable when using the SMMU. This is because we would do 1:1 mapping for the foreign mapping as well.

It might be possible to take advantage of the direct mapping property if Linux do some bookeeping. Although, this wouldn't work for 32-bit Dom0 using short page tables (e.g some version of Debian does) as it may not be able to access all the host RAM. Whether we still care about is a different situation :).

For all the other domains, I think we would want the toolstack to provide a region that can be safely used for foreign mapping (similar to what we already do for the grant-table).

You can still use the map-on-fault behaviour as above, but I would
recommend that you try to limit the number of hypercalls issued.
Having to issue a single hypercall for each page fault it's going to
be slow, so I would instead use mmap batch to map the hole range in
unpopulated physical memory and then the OS fault handler just needs to
fill the page tables with the corresponding address.
IIUC your proposal, you are assuming that you will have enough free space in the physical address space to map the foreign mapping.

However that amount of free space is not unlimited and may be quite small (see above). It would be fairly easy to exhaust it given that a userspace application can map many times the same guest physical address.

So I still think we need to be able to allow Linux to swap a foreign page with another page.


Julien Grall



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