linux-kernel - Re: [PATCH] vdpa/mlx5: Add support for doorbell bypassing

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Date:   Thu, 22 Apr 2021 06:27:02 +0300
From:   Mika Penttilä <mika.penttila@...tfour.com>
To:     Jason Wang <jasowang@...hat.com>, Eli Cohen <elic@...dia.com>,
        mst@...hat.com
Cc:     virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vdpa/mlx5: Add support for doorbell bypassing



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@...dia.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.

--Mika



>
>>       return ret;
>>   }
>