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

[Si-Wei Liu <siwliu.kernel@gmail.com>]

On Sun, Apr 25, 2021 at 7:38 PM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2021/4/25 下午9:25, 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
> >>
> > This nearly matches we have in ConnectX devices. All the doorbells are
> > located at the same place. For 4K page size atchitectures it is aligned
> > to the start of the page. For larger page sizes it is not aligned.
> > If we don't allow to some offset within the page, it means that direct
> > doorbells will not work for 64K page size archs over ConnectX.
>
>
> Right, just to clarify. This can still be model by the current
> page-per-vq model. It means the doorbell will be mapped into different
> pages for each virtqueue by Qemu. So from the view of Qemu or guest,
> each virtqueue has its own doorbell in this case.

So this is the proposed model for mlx5 vdpa with doorbell per-instance
(rather than per-vq), assuming the exclusive ownership of mapped
doorbell page?

>
>
> >
> >> This is not yet supported by Qemu.
>
>
> For "not supported" I meant present this (doorbells sharing) layout to
> guest.

So it means this new layout perhaps will have to introduce new virtio
features to guest thus not compatible with eixsting driver?

-Siwei
>
> Thanks
>
>
> >>
> >> 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().
>