Re: [PATCH v1] vhost-vdpa: Set discarding of RAM broken when initializing the backend

[David Hildenbrand]

On 03.03.21 11:26, David Hildenbrand wrote:
> On 03.03.21 03:53, Jason Wang wrote:
> > On 2021/3/3 12:21 上午, David Hildenbrand wrote:
> > > Similar to VFIO, vDPA will go ahead an map+pin all guest memory. Memory
> > > that used to be discarded will get re-populated and if we
> > > discard+re-access memory after mapping+pinning, the pages mapped into the
> > > vDPA IOMMU will go out of sync with the actual pages mapped into the user
> > > space page tables.
> > >
> > > Set discarding of RAM broken such that:
> > > - virtio-mem and vhost-vdpa run mutually exclusive
> > > - virtio-balloon is inhibited and no memory discards will get issued
> > >
> > > In the future, we might be able to support coordinated discarding of RAM
> > > as used by virtio-mem and as planned for VFIO.
> > >
> > > Cc: Jason Wang <jasowang@redhat.com>
> > > Cc: Michael S. Tsirkin <mst@redhat.com>
> > > Cc: Cindy Lu <lulu@redhat.com>
> > > Signed-off-by: David Hildenbrand <david@redhat.com>
> >
> >
> > Acked-by: Jason Wang <jasowang@redhat.com>
> >
> >
> > > ---
> > >
> > > Note: I was not actually able to reproduce/test as I fail to get the
> > > vdpa_sim/vdpa_sim_net running on upstream Linux (whetever vdpa, vhost_vdpa,
> > > vdpa_sim, vdpa_sim_net modules I probe, and in which order, no vdpa devices
> > > appear under /sys/bus/vdpa/devices/ or /dev/).
> >
> >
> > The device creation was switched to use vdpa tool that is integrated
> > with iproue2[1].
> >
> > [1]
> > https://git.kernel.org/pub/scm/network/iproute2/iproute2-next.git/commit/?id=143610383da51e1f868c6d5a2a5e2fb552293d18
>
> It would be great to document that somewhere if not already done. I only
> found older RH documentations that were not aware of that. I'll give it
> a try - thanks!

Seems to work just fine:


$ sudo ./build/qemu-system-x86_64 -m 2G,maxmem=4G --enable-kvm -object 
memory-backend-ram,id=mem0,size=2G  -device 
virtio-mem-pci,id=vmem0,memdev=mem0,node=0,requested-size=0G  -netdev 
type=vhost-vdpa,vhostdev=/dev/vhost-vdpa-0,id=vhost-vdpa1 -device 
virtio-net-pci,netdev=vhost-vdpa1,mac=00:e8:ca:33:ba:05,disable-modern=off,page-per-vq=on
 -nographic
qemu-system-x86_64: -device 
virtio-mem-pci,id=vmem0,memdev=mem0,node=0,requested-size=0G: Discarding RAM is 
disabled

I think the -netdev is always processed/initialized before the
"-device virtio-mem-pci", which is why we always fail from virtio-mem code
right now and not from vhost-vdpa code.

--
Thanks,

David / dhildenb