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

[Si-Wei Liu]

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...

Regards,
-Siwei
> Thanks