Re: [PATCH] Add shared memory PCI device that shares a memory object betweens VMs

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Anthony Liguori wrote:
I disagree with this. While virtio is excellent at exporting guest memory, it isn't so good at importing another guest's memory.

First we need to separate static memory sharing and dynamic memory sharing. Static memory sharing has to be configured on start up. I think in practice, static memory sharing is not terribly interesting except for maybe embedded environments.

Dynamically memory sharing requires bidirectional communication in order to establish mappings and tear down mappings. You'll eventually recreate virtio once you've implemented this communication mechanism.


I guess that depends on what one uses share memory for.

Cam?

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.

But from the guest's perspective, it's RAM is being used for memory sharing.

If you're clever, you could start a guest with -mem-path and then use this mechanism to map a portion of one guest's memory into another guest without either guest ever knowing who "owns" the memory and with exactly the same driver on both.


If it's part of the normal address space, it will just confuse the guest. Consider for example a reboot.

Shared memory is not normal RAM!

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.

Then you need a side channel to communicate the information to the guest.

There is the PCI config space for that.

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.

It's going to be hard to "fix" with TCG.


Why? Instead of an offset against phys_ram_base you'd store an offset against (char *)0 in the tlb. Where do you see an issue?


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?

It doesn't but it couldn't today either b/c of the 32-bit BARs.

Let's compare the two approaches, not how they fit or don't fit random qemu limitations which need lifting anyway.

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