Date:   Mon, 14 Aug 2023 10:59:16 +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 Fri, Aug 11, 2023 at 6:21 AM Si-Wei Liu <si-wei.liu@...cle.com> wrote:
>
>
>
> On 8/9/2023 8:10 PM, Jason Wang wrote:
> > On Thu, Aug 10, 2023 at 8:40 AM Si-Wei Liu <si-wei.liu@...cle.com> wrote:
> >>
> >>
> >> On 8/8/2023 11:52 PM, Jason Wang wrote:
> >>> 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.
> >> This I agreed. AFAICT QEMU would need to check this new feature flag to
> >> make sure memory mappings are kept intact across reset, otherwise for
> >> the sake of avoid breaking older kernels it has to recreate all the
> >> mappings after reset like how it is done today.
> >>
> >>> 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?
> >> Not sure I understand this proposal. The 1:1 DMA MR is first created at
> >> vdpa device add, and gets destroyed implicitly when the first .set_map
> >> or .dma_map call is made, which is only possible after the vhost-vdpa
> >> module is loaded and bound to vdpa devices.
> > So what happens if there's a buggy userspace that doesn't do any IOTLB setup?
> Then parent driver doesn't do anything in .reset_map() - as the DMA MR
> is still there. Parent driver should be able to tell apart if DMA MR has
> been destroyed or not by checking the internal state.

Would you mind posting a patch to demonstrate this?

Thanks

>
> -Siwei
>
> >
> > Thanks
> >
> >> Naturally the DMA MR should
> >> be restored to how it was before when vhost-vdpa module is unloaded, or
> >> if anything the 1:1 DMA MR creation can be deferred to until virtio-vdpa
> >> is probed and bound to devices. Today vhost_vdpa_remove_as() as part of
> >> the vhost-vdpa unload code path already gets all mappings purged through
> >> vhost_vdpa_iotlb_unmap(0, -1ULL), and it should be pretty safe to
> >> restore DMA MR via .reset_map() right after. Not sure what's the concern
> >> here with buggy vhost-vdpa device?
> >>
> >> Noted when vhost-vdpa is being unloaded there's even no chance to probe
> >> kernel through CVQ, as the virtio feature is not even negotiated at that
> >> point. And it is even trickier to wait for CVQ response from device
> >> indefinitely when trying to unload a module.
> >>
> >> Regards,
> >> -Siwei
> >>> 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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