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Subject: Re: [PATCH] vdpa/mlx5: Add support for doorbell bypassing
To:     Eli Cohen <elic@nvidia.com>
Cc:     mst@redhat.com, virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
References: <20210421104145.115907-1-elic@nvidia.com>
 <e1885255-34f2-9e90-6478-ff0850a5a3d4@redhat.com>
 <20210422060358.GA140698@mtl-vdi-166.wap.labs.mlnx>
 <20210422080725.GB140698@mtl-vdi-166.wap.labs.mlnx>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <9d3d8976-800d-bb14-0a4a-c4b008f6872c@redhat.com>
Date:   Thu, 22 Apr 2021 16:21:45 +0800
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在 2021/4/22 下午4:07, Eli Cohen 写道:
> On Thu, Apr 22, 2021 at 09:03:58AM +0300, Eli Cohen wrote:
>> On Thu, Apr 22, 2021 at 10:37:38AM +0800, 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).
>>>
>> Correct, so I guess I should return here 4096.


I'm not quite sure but since the calculation of the sf_bar_length is 
doen via a shift of 12, it might be correct.

And please double check if the doorbell own the page exclusively.


>>
>> I also think that the check in vhost_vdpa_mmap() should verify that the
>> returned size is not smaller than PAGE_SIZE because the returned address
> Actually I think it's ok since you verify the size equals vma->vm_end -
> vma->vm_start which must be at least PAGE_SIZE.


Yes.

Thanks


>
>> might just be aligned to PAGE_SIZE. I think this should be enoght but
>> maybe also use the same logic in vhost_vdpa_fault().