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

[Zhu, Lingshan]

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@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).
> > > > > > > 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().