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

[Michael S. Tsirkin]

On Wed, Sep 12, 2018 at 11:15:32AM -0600, Alex Williamson wrote:
> 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,

It will violate the requirement of not unmapping part of a region,
won't it? So it won't work.

> the
> ioctls should have used start and end rather than start and size, but
> it's too late to change.  Thanks,
> 
> Alex

At some point we'll have to fix above issue in some way. Maybe when we
add support for atomic remaps.

-- 
MST