Date:   Fri, 30 Apr 2021 21:49:54 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.com>,
        Eli Cohen <elic@...dia.com>
Cc:     mst@...hat.com, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vdpa/mlx5: Add support for doorbell bypassing


在 2021/4/30 下午5:25, Zhu, Lingshan 写道:
>
>
> 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@...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).
>>>>>>>>>>>
>>>>>>>>>> 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. 


The code has already did this, isn't it?

         if (vma->vm_end - vma->vm_start != PAGE_SIZE)
                 return -EINVAL;

...

         notify = ops->get_vq_notification(vdpa, index);
         if (notify.addr & (PAGE_SIZE - 1))
                 return -EINVAL;
         if (vma->vm_end - vma->vm_start != notify.size)
                 return -ENOTSUPP;


> If the doorbell placed at the last 4K in bar, map 64k page could be an 
> error.


mmap() will fail in this case.

Thanks


>
> 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().
>>>>
>>>
>>
>

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