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Subject: Re: [PATCH] vdpa/mlx5: Add support for doorbell bypassing
To:     Jason Wang <jasowang@redhat.com>, 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>
 <9d3d8976-800d-bb14-0a4a-c4b008f6872c@redhat.com>
 <20210422083902.GA146406@mtl-vdi-166.wap.labs.mlnx>
 <bdf10e38-8746-51cf-b460-a904a133329c@redhat.com>
 <20210429100033.GA215200@mtl-vdi-166.wap.labs.mlnx>
 <fc887d99-7058-1057-2d1a-3bdc5802a59a@redhat.com>
 <836263af-6791-0bd3-22c7-22197da021e9@intel.com>
 <79d57f53-a5c9-58df-4a79-6cc7892ab1a2@redhat.com>
From:   "Zhu, Lingshan" <lingshan.zhu@intel.com>
Message-ID: <35c30715-f24b-704c-af3c-2b0259c2fd43@intel.com>
Date:   Fri, 30 Apr 2021 17:25:00 +0800
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On 4/30/2021 3:03 PM, Jason Wang wrote:
>
> 在 2021/4/30 下午2:31, Zhu, Lingshan 写道:
>>
>>
>> On 4/30/2021 12:40 PM, Jason Wang wrote:
>>>
>>> 在 2021/4/29 下午6:00, Eli Cohen 写道:
>>>> On Thu, Apr 22, 2021 at 04:59:11PM +0800, Jason Wang wrote:
>>>>> 在 2021/4/22 下午4:39, Eli Cohen 写道:
>>>>>> On Thu, Apr 22, 2021 at 04:21:45PM +0800, Jason Wang wrote:
>>>>>>> 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 am checking if it is safe to map the any part of the SF's BAR to
>>>>>> userspace without harming other functions. If this is true, I 
>>>>>> will check
>>>>>> if I can return PAGE_SIZE without compromising security.
>>>>>
>>>>> It's usally not safe and a layer violation if other registers are 
>>>>> placed at
>>>>> the same page.
>>>>>
>>>>>
>>>>>>    I think we may
>>>>>> need to extend struct vdpa_notification_area to contain another 
>>>>>> field
>>>>>> offset which indicates the offset from addr where the actual 
>>>>>> doorbell
>>>>>> resides.
>>>>>
>>>>> The movitiaton of the current design is to be fit seamless into 
>>>>> how Qemu
>>>>> model doorbell layouts currently:
>>>>>
>>>>> 1) page-per-vq, each vq has its own page aligned doorbell
>>>>> 2) 2 bytes doorbell, each vq has its own 2 byte aligend doorbell
>>>>>
>>>>> Only 1) is support in vhost-vDPA (and vhost-user) since it's 
>>>>> rather simple
>>>>> and secure (page aligned) to be modelled and implemented via mmap().
>>>>>
>>>>> Exporting a complex layout is possbile but requires careful design.
>>>>>
>>>>> Actually, we had antoher option
>>>>>
>>>>> 3) shared doorbell: all virtqueue shares a single page aligned 
>>>>> doorbell
>>>> I am not sure how this could solve the problem of 64KB archs.
>>>> The point is that in ConnectX devices, the virtio queue objects 
>>>> doorbell
>>>> is aligned to 4K. For larger system page sizes, the doorbell may 
>>>> not be
>>>> aligned to a system page.
>>>> So it seems not too complex to introduce offset within the page.
>>>
>>>
>>> Three major issues:
>>>
>>> 1) single mmap() works at page level, it means we need map 64K to 
>>> guest and we can only do this safely if no other registers are 
>>> placed into the same page
>>> 2) new uAPI to let the userspace know the offset
>>> 3) how to model them with the virtio-pci in Qemu, and this may 
>>> introduce burdens for management (need some changes in the qemu 
>>> command line) to deal with the migration compatibility
>>>
>>> So consider the complexity, we can just stick to the current code. 
>>> That means mmap() will fail and qemu will keep using the eventfd 
>>> based kick.
>> There is another case, mmap() works at page level, page size is at 
>> least 4K. Consider if a device has a bar containing the shared 
>> doorbell page at its last 4K space. In this bar layout, map a 
>> arch.page_size=64K page to usersapce would lead to fatal errors.
>
>
> Why it's a fatal error? Userspace should survive from mmap() errors 
> and keep using the kickfd.
I mean vhost-vdpa should not only check the alignment, also need to 
check whether the doorbell size no less than a arch.page_size. If the 
doorbell placed at the last 4K in bar, map 64k page could be an error.

Thanks
>
>
>> I think we can assign the actual size of the doorbell area size to 
>> vdpa_notification.size than arch.page_size to avoid such issues. Then 
>> upper layers like vhost_vdpa should check whether this size can work 
>> with the machine arch and its alignment, if not, should fail over to 
>> use eventfd.
>
>
> Isn't this how get_vet_notification() designed and implemented right 
> now? What parent need is just to report the doorbell size, it's the 
> bus driver (vhost-vDPA) to decide if and how it is used.
>
> Thanks
>
>
>> Then do we still need a uAPI tell the offset within the page?
>>
>> Thanks
>> Zhu Lingshan
>>>
>>>
>>>
>>>>
>>>> BTW, for now, I am going to send another patch that makes sure page
>>>> boundaries are not vilated. It requires some support from mlx5_core
>>>> which is currently being reviewed internally.
>>>
>>>
>>> Sure.
>>>
>>> Thanks
>>>
>>>
>>>>
>>>>> This is not yet supported by Qemu.
>>>>>
>>>>> Thanks
>>>>>
>>>>>
>>>>>>>>> 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().
>>>
>>
>