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

[Alex Williamson]

On Mon, 15 Feb 2021 15:03:43 +0100
David Hildenbrand <david@redhat.com> wrote:

> 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.

Sorry for the delay.  It looks to me like patches 1, 8, and 9 are
Memory API that are still missing an Ack from Paolo.  I'll toss in my
A-b+R-b for patches 6 and 7.  I don't see that this necessarily needs
to go in through vfio, I'm more than happy if someone else wants to
grab it.  Thanks,

Alex