Re: [PATCH RFC 4/4] vhost-vdpa: introduce IOTLB_PERSIST backend feature bit

[Jason Wang]

On Wed, Aug 16, 2023 at 6:31 AM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote:
>
>
>
> On 8/14/2023 7:25 PM, Jason Wang wrote:
> > On Tue, Aug 15, 2023 at 9:45 AM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote:
> >> Signed-off-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
> >> ---
> >>   drivers/vhost/vdpa.c             | 16 +++++++++++++++-
> >>   include/uapi/linux/vhost_types.h |  2 ++
> >>   2 files changed, 17 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> >> index 62b0a01..75092a7 100644
> >> --- a/drivers/vhost/vdpa.c
> >> +++ b/drivers/vhost/vdpa.c
> >> @@ -406,6 +406,14 @@ static bool vhost_vdpa_can_resume(const struct vhost_vdpa *v)
> >>          return ops->resume;
> >>   }
> >>
> >> +static bool vhost_vdpa_has_persistent_map(const struct vhost_vdpa *v)
> >> +{
> >> +       struct vdpa_device *vdpa = v->vdpa;
> >> +       const struct vdpa_config_ops *ops = vdpa->config;
> >> +
> >> +       return (!ops->set_map && !ops->dma_map) || ops->reset_map;
> > So this means the IOTLB/IOMMU mappings have already been decoupled
> > from the vdpa reset.
> Not in the sense of API, it' been coupled since day one from the
> implementations of every on-chip IOMMU parent driver, namely mlx5_vdpa
> and vdpa_sim. Because of that, later on the (improper) support for
> virtio-vdpa, from commit 6f5312f80183 ("vdpa/mlx5: Add support for
> running with virtio_vdpa") and 6c3d329e6486 ("vdpa_sim: get rid of DMA
> ops") misused the .reset() op to realize 1:1 mapping, rendering strong
> coupling between device reset and reset of iotlb mappings. This series
> try to rectify that implementation deficiency, while keep userspace
> continuing to work with older kernel behavior.
>
> >   So it should have been noticed by the userspace.
> Yes, userspace had noticed this no-chip IOMMU discrepancy since day one
> I suppose. Unfortunately there's already code in userspace with this
> assumption in mind that proactively tears down and sets up iotlb mapping
> around vdpa device reset...
> > I guess we can just fix the simulator and mlx5 then we are fine?
> Only IF we don't care about running new QEMU on older kernels with
> flawed on-chip iommu behavior around reset. But that's a big IF...

So what I meant is:

Userspace doesn't know whether the vendor specific mappings (set_map)
are required or not. And in the implementation of vhost_vdpa, if
platform IOMMU is used, the mappings are decoupled from the reset. So
if the Qemu works with parents with platform IOMMU it means Qemu can
work if we just decouple vendor specific mappings from the parents
that uses set_map.

Thanks

>
> Regards,
> -Siwei
> >
> > Thanks
> >
>