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

From: Jason Wang
Date: Fri Apr 30 2021 - 00:40:27 EST


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.



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().