Re: [PATCH v1 0/9] virtio-mem: vfio support

[David Hildenbrand]

On 19.11.20 16:39, David Hildenbrand wrote:
> This is the follow-up of:
>   https://lkml.kernel.org/r/20200924160423.106747-1-david@redhat.com
> to make vfio and virtio-mem play together. The basic idea was the result of
> Alex brainstorming with me on how to tackle this.
> 
> A virtio-mem device manages a memory region in guest physical address
> space, represented as a single (currently large) memory region in QEMU,
> mapped into system memory address space. Before the guest is allowed to use
> memory blocks, it must coordinate with the hypervisor (plug blocks). After
> a reboot, all memory is usually unplugged - when the guest comes up, it
> detects the virtio-mem device and selects memory blocks to plug (based on
> resize requests from the hypervisor).
> 
> Memory hot(un)plug consists of (un)plugging memory blocks via a virtio-mem
> device (triggered by the guest). When unplugging blocks, we discard the
> memory - similar to memory balloon inflation. In contrast to memory
> ballooning, we always know which memory blocks a guest may actually use -
> especially during a reboot, after a crash, or after kexec (and during
> hibernation as well). Guests agreed to no access unplugged memory again,
> especially not via DMA.
> 
> The issue with vfio is, that it cannot deal with random discards - for this
> reason, virtio-mem and vfio can currently only run mutually exclusive.
> Especially, vfio would currently map the whole memory region (with possible
> only little/no plugged blocks), resulting in all pages getting pinned and
> therefore resulting in a higher memory consumption than expected (turning
> virtio-mem basically useless in these environments).
> 
> To make vfio work nicely with virtio-mem, we have to map only the plugged
> blocks, and map/unmap properly when plugging/unplugging blocks (including
> discarding of RAM when unplugging). We achieve that by using a new notifier
> mechanism that communicates changes.
> 
> It's important to map memory in the granularity in which we could see
> unmaps again (-> virtio-mem block size) - so when e.g., plugging
> consecutive 100 MB with a block size of 2MB, we need 50 mappings. When
> unmapping, we can use a single vfio_unmap call for the applicable range.
> We expect that the block size of virtio-mem devices will be fairly large
> in the future (to not run out of mappings and to improve hot(un)plug
> performance), configured by the user, when used with vfio (e.g., 128MB,
> 1G, ...).
> 
> More info regarding virtio-mem can be found at:
>     https://virtio-mem.gitlab.io/
> I'll add a guide for virtio-mem+vfio soonish.

There is now a guide/example at:

https://virtio-mem.gitlab.io/user-guide/user-guide-qemu.html#vfio-vfio-pci


-- 
Thanks,

David / dhildenb