Message-ID: <CACGkMEsj=Bg3GvsErWQqe0Ov8CqT9un0nTFeDYW6EzzowJm9rQ@mail.gmail.com>
Date:   Wed, 9 Aug 2023 14:52:05 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Si-Wei Liu <si-wei.liu@...cle.com>
Cc:     Dragos Tatulea <dtatulea@...dia.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Gal Pressman <gal@...dia.com>, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Eugenio Pérez <eperezma@...hat.com>
Subject: Re: [PATCH 1/2] vdpa/mlx5: Fix mr->initialized semantics

On Wed, Aug 9, 2023 at 6:58 AM Si-Wei Liu <si-wei.liu@...cle.com> wrote:
>
>
>
> 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@...cle.com> wrote:
> >>
> >>
> >> On 8/3/2023 1:03 AM, Jason Wang wrote:
> >>> On Thu, Aug 3, 2023 at 1:13 AM Dragos Tatulea <dtatulea@...dia.com> 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@...dia.com>
> >>>> Reviewed-by: Eugenio Pérez <eperezma@...hat.com>
> >>>> Reviewed-by: Gal Pressman <gal@...dia.com>
> >>>> ---
> >>>>    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@nongnu.org/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?

Ok, I think I miss some context. I agree that it's better to decouple
memory mappings from the virtio reset. It helps to reduce the
unnecessary memory transactions. It might require a new feature flag.

Regarding the method of restoring to 1:1 DMA MR, it might be dangerous
for (buggy) vhost-vDPA devices. Since its userspace doesn't set up any
mapping it can explore the kernel with that via CVQ?

Thanks

>
> >   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@...dia.com>
> >>> 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@...dia.com>
> >>>       Link: https://lore.kernel.org/r/20210602085854.62690-1-elic@nvidia.com
> >>>       Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> >>>       Acked-by: Jason Wang <jasowang@...hat.com>
> >>>
> >>> 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
> >>>>
> >>> _______________________________________________
> >>> Virtualization mailing list
> >>> Virtualization@...ts.linux-foundation.org
> >>> https://lists.linuxfoundation.org/mailman/listinfo/virtualization
>

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