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Subject: Re: [PATCH] vdpa/mlx5: Add support for doorbell bypassing
To:     =?UTF-8?Q?Mika_Penttil=c3=a4?= <mika.penttila@nextfour.com>,
        Eli Cohen <elic@nvidia.com>, mst@redhat.com
Cc:     virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
References: <20210421104145.115907-1-elic@nvidia.com>
 <e1885255-34f2-9e90-6478-ff0850a5a3d4@redhat.com>
 <a77efb06-b2ae-c94a-96bc-290b8b7332ba@nextfour.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <7bdb251a-5e16-dd22-d204-6ff57e12e72f@redhat.com>
Date:   Thu, 22 Apr 2021 12:52:09 +0800
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在 2021/4/22 上午11:27, Mika Penttilä 写道:
>
>
> On 22.4.2021 5.37, Jason Wang wrote:
>>
>> 在 2021/4/21 下午6:41, Eli Cohen 写道:
>>> Implement mlx5_get_vq_notification() to return the doorbell address.
>>> Size is set to one system page as required.
>>>
>>> Signed-off-by: Eli Cohen <elic@nvidia.com>
>>> ---
>>>   drivers/vdpa/mlx5/core/mlx5_vdpa.h | 1 +
>>>   drivers/vdpa/mlx5/core/resources.c | 1 +
>>>   drivers/vdpa/mlx5/net/mlx5_vnet.c  | 6 ++++++
>>>   3 files changed, 8 insertions(+)
>>>
>>> diff --git a/drivers/vdpa/mlx5/core/mlx5_vdpa.h 
>>> b/drivers/vdpa/mlx5/core/mlx5_vdpa.h
>>> index b6cc53ba980c..49de62cda598 100644
>>> --- a/drivers/vdpa/mlx5/core/mlx5_vdpa.h
>>> +++ b/drivers/vdpa/mlx5/core/mlx5_vdpa.h
>>> @@ -41,6 +41,7 @@ struct mlx5_vdpa_resources {
>>>       u32 pdn;
>>>       struct mlx5_uars_page *uar;
>>>       void __iomem *kick_addr;
>>> +    u64 phys_kick_addr;
>>>       u16 uid;
>>>       u32 null_mkey;
>>>       bool valid;
>>> diff --git a/drivers/vdpa/mlx5/core/resources.c 
>>> b/drivers/vdpa/mlx5/core/resources.c
>>> index 6521cbd0f5c2..665f8fc1710f 100644
>>> --- a/drivers/vdpa/mlx5/core/resources.c
>>> +++ b/drivers/vdpa/mlx5/core/resources.c
>>> @@ -247,6 +247,7 @@ int mlx5_vdpa_alloc_resources(struct 
>>> mlx5_vdpa_dev *mvdev)
>>>           goto err_key;
>>>         kick_addr = mdev->bar_addr + offset;
>>> +    res->phys_kick_addr = kick_addr;
>>>         res->kick_addr = ioremap(kick_addr, PAGE_SIZE);
>>>       if (!res->kick_addr) {
>>> diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c 
>>> b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> index 10c5fef3c020..680751074d2a 100644
>>> --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
>>> @@ -1865,8 +1865,14 @@ static void mlx5_vdpa_free(struct vdpa_device 
>>> *vdev)
>>>     static struct vdpa_notification_area 
>>> mlx5_get_vq_notification(struct vdpa_device *vdev, u16 idx)
>>>   {
>>> +    struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
>>>       struct vdpa_notification_area ret = {};
>>> +    struct mlx5_vdpa_net *ndev;
>>> +
>>> +    ndev = to_mlx5_vdpa_ndev(mvdev);
>>>   +    ret.addr = (phys_addr_t)ndev->mvdev.res.phys_kick_addr;
>>> +    ret.size = PAGE_SIZE;
>>
>>
>> Note that the page will be mapped in to guest, so it's only safe if 
>> the doorbeel exclusively own the page. This means if there're other 
>> registers in the page, we can not let the doorbell bypass to work.
>>
>> So this is suspicious at least in the case of subfunction where we 
>> calculate the bar length in mlx5_sf_dev_table_create() as:
>>
>> table->sf_bar_length = 1 << (MLX5_CAP_GEN(dev, log_min_sf_size) + 12);
>>
>> It looks to me this can only work for the arch with PAGE_SIZE = 4096, 
>> otherwise we can map more into the userspace(guest).
>>
>> Thanks
>>
>>
> Is there support as of today (in qemu  or elsewhere) to use this mmap 
> doorbell instead of the traditional kick.


Vhost-user had already had this support in Qemu (see set_host_notifier_mr).

And I've posted patches to support this for qemu[1] and vp_vdpa[2] driver.

Thanks

[1] https://www.mail-archive.com/qemu-devel@nongnu.org/msg798935.html

[2] 
https://lore.kernel.org/virtualization/20210415073147.19331-5-jasowang@redhat.com/T/


>
> --Mika
>
>
>
>>
>>>       return ret;
>>>   }
>>
>