Message-ID: <CACGkMEtWsBRtW3DNQudcWJEtRb7isG-yRvvL3kL2L7k_=TgkZQ@mail.gmail.com>
Date:   Thu, 10 Aug 2023 11:10:39 +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 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?

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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