Re: [PATCH v5 00/11] virtio-mem: vfio support

[David Hildenbrand <david@redhat.com>]

On 27.01.21 13:45, Michael S. Tsirkin wrote:
> On Thu, Jan 21, 2021 at 12:05:29PM +0100, David Hildenbrand wrote:
>> 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 not 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.
> 
> series
> 
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> 
> virtio bits
> 
> Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> 
> This needs to go through vfio tree I assume.

Thanks Michael.

@Alex, what are your suggestions?


-- 
Thanks,

David / dhildenb