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

[David Hildenbrand <david@redhat.com>]

On 08.02.21 09:28, David Hildenbrand wrote:
> 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?

Gentle ping.


-- 
Thanks,

David / dhildenb