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Date:   Thu, 10 Aug 2023 15:20:51 -0700
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Subject: Re: [PATCH 1/2] vdpa/mlx5: Fix mr->initialized semantics
Content-Language: en-US
To:     Jason Wang <jasowang@redhat.com>
Cc:     Dragos Tatulea <dtatulea@nvidia.com>,
        Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Gal Pressman <gal@nvidia.com>, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        =?UTF-8?Q?Eugenio_P=c3=a9rez?= <eperezma@redhat.com>
References: <20230802171231.11001-1-dtatulea@nvidia.com>
 <20230802171231.11001-3-dtatulea@nvidia.com>
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 <3472a482-dc1d-1994-e2d7-c2bd740bbd27@oracle.com>
 <CACGkMEsj=Bg3GvsErWQqe0Ov8CqT9un0nTFeDYW6EzzowJm9rQ@mail.gmail.com>
 <e5a853f1-d83b-ffe5-b96a-b3c0d9c6f528@oracle.com>
 <CACGkMEtWsBRtW3DNQudcWJEtRb7isG-yRvvL3kL2L7k_=TgkZQ@mail.gmail.com>
From:   Si-Wei Liu <si-wei.liu@oracle.com>
Organization: Oracle Corporation
In-Reply-To: <CACGkMEtWsBRtW3DNQudcWJEtRb7isG-yRvvL3kL2L7k_=TgkZQ@mail.gmail.com>
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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@oracle.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@oracle.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@oracle.com> wrote:
>>>>>> On 8/3/2023 1:03 AM, Jason Wang wrote:
>>>>>>> On Thu, Aug 3, 2023 at 1:13 AM Dragos Tatulea <dtatulea@nvidia.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@nvidia.com>
>>>>>>>> Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
>>>>>>>> Reviewed-by: Gal Pressman <gal@nvidia.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.

-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@nvidia.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@nvidia.com>
>>>>>>>         Link: https://lore.kernel.org/r/20210602085854.62690-1-elic@nvidia.com
>>>>>>>         Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>>>>>>>         Acked-by: Jason Wang <jasowang@redhat.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
>>>>>>>>
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