Anthony Liguori wrote:
Hi Cam,
Cam Macdonell wrote:
This patch supports sharing memory between VMs and between the
host/VM. It's a first cut and comments are encouraged. The goal is
to support simple Inter-VM communication
with zero-copy access to shared memory.
Nice work!
I would suggest two design changes to make here. The first is that I
think you should use virtio.
I disagree with this. While virtio is excellent at exporting guest
memory, it isn't so good at importing another guest's memory.
The second is that I think instead of relying on mapping in device
memory to the guest, you should have the guest allocate it's own
memory to dedicate to sharing.
That's not what you describe below. You're having the guest allocate
parts of its address space that happen to be used by RAM, and overlaying
those parts with the shared memory.
Right now, you've got a bit of a hole in your implementation because
you only support files that are powers-of-two in size even though
that's not documented/enforced. This is a limitation of PCI resource
regions.
While the BAR needs to be a power of two, I don't think the RAM backing
it needs to be.
Also, the PCI memory hole is limited in size today which is going to
put an upper bound on the amount of memory you could ever map into a
guest.
Today. We could easily lift this restriction by supporting 64-bit
BARs. It would probably take only a few lines of code.
Since you're using qemu_ram_alloc() also, it makes hotplug unworkable
too since qemu_ram_alloc() is a static allocation from a contiguous heap.
We need to fix this anyway, for memory hotplug.
If you used virtio, what you could do is provide a ring queue that was
used to communicate a series of requests/response. The exchange might
look like this:
guest: REQ discover memory region
host: RSP memory region id: 4 size: 8k
guest: REQ map region id: 4 size: 8k: sgl: {(addr=43000, size=4k),
(addr=944000,size=4k)}
host: RSP mapped region id: 4
guest: REQ notify region id: 4
host: RSP notify region id: 4
guest: REQ poll region id: 4
host: RSP poll region id: 4
That looks significantly more complex.
And the REQ/RSP order does not have to be in series like this. In
general, you need one entry on the queue to poll for new memory
regions, one entry for each mapped region to poll for incoming
notification, and then the remaining entries can be used to send
short-lived requests/responses.
It's important that the REQ map takes a scatter/gather list of
physical addresses because after running for a while, it's unlikely
that you'll be able to allocate any significant size of contiguous
memory.
From a QEMU perspective, you would do memory sharing by waiting for a
map REQ from the guest and then you would complete the request by
doing an mmap(MAP_FIXED) with the appropriate parameters into
phys_ram_base.
That will fragment the vma list. And what do you do when you unmap the
region?
How does a 256M guest map 1G of shared memory?
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
