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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>
From:   "Zhu, Lingshan" <lingshan.zhu@intel.com>
Message-ID: <836263af-6791-0bd3-22c7-22197da021e9@intel.com>
Date:   Fri, 30 Apr 2021 14:31:23 +0800
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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.
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.
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().
>