Re: [Qemu-devel] [RFC 3/3] vhost-user: support programming VFIO group in master

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 12 Sep 2018 12:14:44 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, Sep 12, 2018 at 04:04:00PM +0800, Tiwei Bie wrote:
> > On Mon, Jul 30, 2018 at 12:30:58PM +0300, Michael S. Tsirkin wrote:
> > [...]  
> > > 
> > > I generally wonder how are restarts of the backend handled
> > > with this approach: closing the VFIO device tends to reset
> > > the whole device.  
> > 
> > Hi Michael,
> > 
> > I missed this comment previously.. This is a good point!
> > In this RFC, before sending the VFIO group fd to QEMU,
> > backend needs to close the VFIO device and unset the VFIO
> > container first. Otherwise, QEMU won't be able to set the
> > VFIO container for the VFIO group.
> > 
> > Another option is to share the container fd instead of
> > the group fd to QEMU. In this case, backend won't need
> > to close any fd. But there is one problem that, it's
> > hard to unmap the old mappings, especially when QEMU
> > crashes.  
> 
> What are these old mappings and who creates them?
> If you want to just reset everything the way it was
> on open, surely it would be easy to add such a reset ioctl.
> 
> > Do you have any suggestions? Thanks!
> > 
> > Best regards,
> > Tiwei Bie  
> 
> Donnu. Alex, any thoughts? Which approach would you prefer?

The existing UNMAP_DMA ioctl for the vfio type1 IOMMU only requires
that an unmap does not bisect previous mappings, ie. a previous mapping
cannot be partially unmapped.  Therefore you can already dump the
entire IOVA space for a container with one UNMAP_DMA call, iova = 0,
size = (u64)-1.  Thanks,

Alex