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

[Alex Williamson]

On Wed, 12 Sep 2018 12:44:15 -0400
"Michael S. Tsirkin" <address@hidden> wrote:

> 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.

Yes, technically you'd need another ioctl to get the last byte, the
ioctls should have used start and end rather than start and size, but
it's too late to change.  Thanks,

Alex