Re: [PATCH 1/2] vdpa/mlx5: Fix mr->initialized semantics

[Si-Wei Liu]

On 8/7/2023 8:00 PM, Jason Wang wrote:
> On Fri, Aug 4, 2023 at 1:58 AM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote:
> > On 8/3/2023 1:03 AM, Jason Wang wrote:
> > > On Thu, Aug 3, 2023 at 1:13 AM Dragos Tatulea <dtatulea@xxxxxxxxxx> wrote:
> > > > The mr->initialized flag is shared between the control vq and data vq
> > > > part of the mr init/uninit. But if the control vq and data vq get placed
> > > > in different ASIDs, it can happen that initializing the control vq will
> > > > prevent the data vq mr from being initialized.
> > > >
> > > > This patch consolidates the control and data vq init parts into their
> > > > own init functions. The mr->initialized will now be used for the data vq
> > > > only. The control vq currently doesn't need a flag.
> > > >
> > > > The uninitializing part is also taken care of: mlx5_vdpa_destroy_mr got
> > > > split into data and control vq functions which are now also ASID aware.
> > > >
> > > > Fixes: 8fcd20c30704 ("vdpa/mlx5: Support different address spaces for control and data")
> > > > Signed-off-by: Dragos Tatulea <dtatulea@xxxxxxxxxx>
> > > > Reviewed-by: Eugenio Pérez <eperezma@xxxxxxxxxx>
> > > > Reviewed-by: Gal Pressman <gal@xxxxxxxxxx>
> > > > ---
> > > >    drivers/vdpa/mlx5/core/mlx5_vdpa.h |  1 +
> > > >    drivers/vdpa/mlx5/core/mr.c        | 97 +++++++++++++++++++++---------
> > > >    2 files changed, 71 insertions(+), 27 deletions(-)
> > > >
> > > > diff --git a/drivers/vdpa/mlx5/core/mlx5_vdpa.h b/drivers/vdpa/mlx5/core/mlx5_vdpa.h
> > > > index 25fc4120b618..a0420be5059f 100644
> > > > --- a/drivers/vdpa/mlx5/core/mlx5_vdpa.h
> > > > +++ b/drivers/vdpa/mlx5/core/mlx5_vdpa.h
> > > > @@ -31,6 +31,7 @@ struct mlx5_vdpa_mr {
> > > >           struct list_head head;
> > > >           unsigned long num_directs;
> > > >           unsigned long num_klms;
> > > > +       /* state of dvq mr */
> > > >           bool initialized;
> > > >
> > > >           /* serialize mkey creation and destruction */
> > > > diff --git a/drivers/vdpa/mlx5/core/mr.c b/drivers/vdpa/mlx5/core/mr.c
> > > > index 03e543229791..4ae14a248a4b 100644
> > > > --- a/drivers/vdpa/mlx5/core/mr.c
> > > > +++ b/drivers/vdpa/mlx5/core/mr.c
> > > > @@ -489,60 +489,103 @@ static void destroy_user_mr(struct mlx5_vdpa_dev *mvdev, struct mlx5_vdpa_mr *mr
> > > >           }
> > > >    }
> > > >
> > > > -void mlx5_vdpa_destroy_mr(struct mlx5_vdpa_dev *mvdev)
> > > > +static void _mlx5_vdpa_destroy_cvq_mr(struct mlx5_vdpa_dev *mvdev, unsigned int asid)
> > > > +{
> > > > +       if (mvdev->group2asid[MLX5_VDPA_CVQ_GROUP] != asid)
> > > > +               return;
> > > > +
> > > > +       prune_iotlb(mvdev);
> > > > +}
> > > > +
> > > > +static void _mlx5_vdpa_destroy_dvq_mr(struct mlx5_vdpa_dev *mvdev, unsigned int asid)
> > > >    {
> > > >           struct mlx5_vdpa_mr *mr = &mvdev->mr;
> > > >
> > > > -       mutex_lock(&mr->mkey_mtx);
> > > > +       if (mvdev->group2asid[MLX5_VDPA_DATAVQ_GROUP] != asid)
> > > > +               return;
> > > > +
> > > >           if (!mr->initialized)
> > > > -               goto out;
> > > > +               return;
> > > >
> > > > -       prune_iotlb(mvdev);
> > > >           if (mr->user_mr)
> > > >                   destroy_user_mr(mvdev, mr);
> > > >           else
> > > >                   destroy_dma_mr(mvdev, mr);
> > > >
> > > >           mr->initialized = false;
> > > > -out:
> > > > +}
> > > > +
> > > > +static void mlx5_vdpa_destroy_mr_asid(struct mlx5_vdpa_dev *mvdev, unsigned int asid)
> > > > +{
> > > > +       struct mlx5_vdpa_mr *mr = &mvdev->mr;
> > > > +
> > > > +       mutex_lock(&mr->mkey_mtx);
> > > > +
> > > > +       _mlx5_vdpa_destroy_dvq_mr(mvdev, asid);
> > > > +       _mlx5_vdpa_destroy_cvq_mr(mvdev, asid);
> > > > +
> > > >           mutex_unlock(&mr->mkey_mtx);
> > > >    }
> > > >
> > > > -static int _mlx5_vdpa_create_mr(struct mlx5_vdpa_dev *mvdev,
> > > > -                               struct vhost_iotlb *iotlb, unsigned int asid)
> > > > +void mlx5_vdpa_destroy_mr(struct mlx5_vdpa_dev *mvdev)
> > > > +{
> > > > +       mlx5_vdpa_destroy_mr_asid(mvdev, mvdev->group2asid[MLX5_VDPA_CVQ_GROUP]);
> > > > +       mlx5_vdpa_destroy_mr_asid(mvdev, mvdev->group2asid[MLX5_VDPA_DATAVQ_GROUP]);
> > > > +}
> > > > +
> > > > +static int _mlx5_vdpa_create_cvq_mr(struct mlx5_vdpa_dev *mvdev,
> > > > +                                   struct vhost_iotlb *iotlb,
> > > > +                                   unsigned int asid)
> > > > +{
> > > > +       if (mvdev->group2asid[MLX5_VDPA_CVQ_GROUP] != asid)
> > > > +               return 0;
> > > > +
> > > > +       return dup_iotlb(mvdev, iotlb);
> > > This worries me as conceptually, there should be no difference between
> > > dvq mr and cvq mr. The virtqueue should be loosely coupled with mr.
> > >
> > > One example is that, if we only do dup_iotlb() but not try to create
> > > dma mr here, we will break virtio-vdpa:
> > For this case, I guess we may need another way to support virtio-vdpa
> > 1:1 mapping rather than overloading virtio device reset semantics, see:
> >
> > https://www.mail-archive.com/qemu-devel@xxxxxxxxxx/msg953755.html
> >
> >   > Conceptually, the address mapping is not a part of the abstraction for
> >   > a virtio device now. So resetting the memory mapping during virtio
> >   > device reset seems wrong.
> >
> > where we want to keep memory mapping intact across virtio device reset
> > for best live migration latency/downtime. I wonder would it work to
> > reset the mapping in vhost-vdpa life cycle out of virtio reset, say
> > introduce a .reset_map() op to restore 1:1 mapping within
> > vhost_vdpa_remove_as() right after vhost_vdpa_iotlb_unmap()? Then we can
> > move the iotlb reset logic to there without worry breaking virtio-vdpa.
> It looks to me we don't need a new ops. We can simply do set_map()
> twice
What does it mean, first set_map(0, -1ULL) with zero iotlb entry passed 
in to destroy all iotlb mappings previously added, and second set_map(0, 
-1ULL) to restore 1:1 DMA MR? But userspace (maybe a buggy one but 
doesn't do harm) apart from vhost-vdpa itself can do unmap twice anyway, 
this is supported today I think. Why there'll be such obscure 
distinction, or what's the benefit to treat second .set_map() as 
recreating 1:1 mapping?

>   or do you mean it would be faster?
I think with .reset_map() we at least can avoid indefinite latency 
hiccup from destroying and recreating 1:1 mapping with the unwarranted 
2rd unmap call. And .reset_map() should work with both .dma_map() and 
.set_map() APIs with clear semantics.

Regards,
-Siwei
> Thanks
>
> > Thanks,
> > -Siwei
> >
> > > commit 6f5312f801836e6af9bcbb0bdb44dc423e129206
> > > Author: Eli Cohen <elic@xxxxxxxxxx>
> > > Date:   Wed Jun 2 11:58:54 2021 +0300
> > >
> > >       vdpa/mlx5: Add support for running with virtio_vdpa
> > >
> > >       In order to support running vdpa using vritio_vdpa driver, we need  to
> > >       create a different kind of MR, one that has 1:1 mapping, since the
> > >       addresses referring to virtqueues are dma addresses.
> > >
> > >       We create the 1:1 MR in mlx5_vdpa_dev_add() only in case firmware
> > >       supports the general capability umem_uid_0. The reason for that is that
> > >       1:1 MRs must be created with uid == 0 while virtqueue objects can be
> > >       created with uid == 0 only when the firmware capability is on.
> > >
> > >       If the set_map() callback is called with new translations provided
> > >       through iotlb, the driver will destroy the 1:1 MR and create a regular
> > >       one.
> > >
> > >       Signed-off-by: Eli Cohen <elic@xxxxxxxxxx>
> > >       Link: https://lore.kernel.org/r/20210602085854.62690-1-elic@xxxxxxxxxx
> > >       Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > >       Acked-by: Jason Wang <jasowang@xxxxxxxxxx>
> > >
> > > Thanks
> > >
> > >
> > > > +}
> > > > +
> > > > +static int _mlx5_vdpa_create_dvq_mr(struct mlx5_vdpa_dev *mvdev,
> > > > +                                   struct vhost_iotlb *iotlb,
> > > > +                                   unsigned int asid)
> > > >    {
> > > >           struct mlx5_vdpa_mr *mr = &mvdev->mr;
> > > >           int err;
> > > >
> > > > -       if (mr->initialized)
> > > > +       if (mvdev->group2asid[MLX5_VDPA_DATAVQ_GROUP] != asid)
> > > >                   return 0;
> > > >
> > > > -       if (mvdev->group2asid[MLX5_VDPA_DATAVQ_GROUP] == asid) {
> > > > -               if (iotlb)
> > > > -                       err = create_user_mr(mvdev, iotlb);
> > > > -               else
> > > > -                       err = create_dma_mr(mvdev, mr);
> > > > +       if (mr->initialized)
> > > > +               return 0;
> > > >
> > > > -               if (err)
> > > > -                       return err;
> > > > -       }
> > > > +       if (iotlb)
> > > > +               err = create_user_mr(mvdev, iotlb);
> > > > +       else
> > > > +               err = create_dma_mr(mvdev, mr);
> > > >
> > > > -       if (mvdev->group2asid[MLX5_VDPA_CVQ_GROUP] == asid) {
> > > > -               err = dup_iotlb(mvdev, iotlb);
> > > > -               if (err)
> > > > -                       goto out_err;
> > > > -       }
> > > > +       if (err)
> > > > +               return err;
> > > >
> > > >           mr->initialized = true;
> > > > +
> > > > +       return 0;
> > > > +}
> > > > +
> > > > +static int _mlx5_vdpa_create_mr(struct mlx5_vdpa_dev *mvdev,
> > > > +                               struct vhost_iotlb *iotlb, unsigned int asid)
> > > > +{
> > > > +       int err;
> > > > +
> > > > +       err = _mlx5_vdpa_create_dvq_mr(mvdev, iotlb, asid);
> > > > +       if (err)
> > > > +               return err;
> > > > +
> > > > +       err = _mlx5_vdpa_create_cvq_mr(mvdev, iotlb, asid);
> > > > +       if (err)
> > > > +               goto out_err;
> > > > +
> > > >           return 0;
> > > >
> > > >    out_err:
> > > > -       if (mvdev->group2asid[MLX5_VDPA_DATAVQ_GROUP] == asid) {
> > > > -               if (iotlb)
> > > > -                       destroy_user_mr(mvdev, mr);
> > > > -               else
> > > > -                       destroy_dma_mr(mvdev, mr);
> > > > -       }
> > > > +       _mlx5_vdpa_destroy_dvq_mr(mvdev, asid);
> > > >
> > > >           return err;
> > > >    }
> > > > --
> > > > 2.41.0
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