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

[Jason Wang]

在 2021/4/22 上午11:27, Mika Penttilä 写道:
> On 22.4.2021 5.37, 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@xxxxxxxxxx>
> > > ---
> > >   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).
> >
> > Thanks
> Is there support as of today (in qemu  or elsewhere) to use this mmap 
> doorbell instead of the traditional kick.


Vhost-user had already had this support in Qemu (see set_host_notifier_mr).

And I've posted patches to support this for qemu[1] and vp_vdpa[2] driver.

Thanks

[1] https://www.mail-archive.com/qemu-devel@xxxxxxxxxx/msg798935.html

[2] 
https://lore.kernel.org/virtualization/20210415073147.19331-5-jasowang@xxxxxxxxxx/T/


> --Mika
>
>
>
> > >       return ret;
> > >   }