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

[Michael S. Tsirkin]

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?

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MST