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

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Date:   Thu, 22 Apr 2021 09:03:58 +0300
From:   Eli Cohen <elic@...dia.com>
To:     Jason Wang <jasowang@...hat.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

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 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
might just be aligned to PAGE_SIZE. I think this should be enoght but
maybe also use the same logic in vhost_vdpa_fault().