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

[Michael S. Tsirkin]

On Wed, Sep 12, 2018 at 10:34:43AM -0600, Alex Williamson wrote:
> On Wed, 12 Sep 2018 12:14:44 -0400
> "Michael S. Tsirkin" <address@hidden> 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

Hmm this would exclude the last byte (address (u64)-1).
VTD does not support such iova values for now but something
to keep in mind e.g. for virtio-iommu with nested virt
which does.

-- 
MST