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Date:   Thu, 22 Apr 2021 11:39:02 +0300
From:   Eli Cohen <elic@nvidia.com>
To:     Jason Wang <jasowang@redhat.com>
CC:     <mst@redhat.com>, <virtualization@lists.linux-foundation.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] vdpa/mlx5: Add support for doorbell bypassing
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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. 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.
> 
> 
> > > 
> > > 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().
>