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Re: [Qemu-devel] [RFC 3/3] vhost-user: support programming VFIO group in
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [RFC 3/3] vhost-user: support programming VFIO group in master |
Date: |
Wed, 12 Sep 2018 13:29:33 -0400 |
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
- Re: [Qemu-devel] [RFC 3/3] vhost-user: support programming VFIO group in master, Tiwei Bie, 2018/09/12
- Re: [Qemu-devel] [RFC 3/3] vhost-user: support programming VFIO group in master, Michael S. Tsirkin, 2018/09/12
- Re: [Qemu-devel] [RFC 3/3] vhost-user: support programming VFIO group in master, Alex Williamson, 2018/09/12
- Re: [Qemu-devel] [RFC 3/3] vhost-user: support programming VFIO group in master, Michael S. Tsirkin, 2018/09/12
- Re: [Qemu-devel] [RFC 3/3] vhost-user: support programming VFIO group in master, Alex Williamson, 2018/09/12
- Re: [Qemu-devel] [RFC 3/3] vhost-user: support programming VFIO group in master,
Michael S. Tsirkin <=
- Re: [Qemu-devel] [RFC 3/3] vhost-user: support programming VFIO group in master, Alex Williamson, 2018/09/12
- Re: [Qemu-devel] [RFC 3/3] vhost-user: support programming VFIO group in master, Tian, Kevin, 2018/09/13