[PATCH net-next v2 1/3] xsk: Remove non-zero 'dma_page' check in xp_assign_dev

[PATCH net-next v2 1/3] xsk: Remove non-zero 'dma_page' check in xp_assign_dev

[Yunjian Wang]

Now dma mappings are used by the physical NICs. However the vNIC
maybe do not need them. So remove non-zero 'dma_page' check in
xp_assign_dev.

Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
---
 net/xdp/xsk_buff_pool.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
index ce60ecd48a4d..a5af75b1f43c 100644
--- a/net/xdp/xsk_buff_pool.c
+++ b/net/xdp/xsk_buff_pool.c
@@ -219,16 +219,9 @@ int xp_assign_dev(struct xsk_buff_pool *pool,
 	if (err)
 		goto err_unreg_pool;
 
-	if (!pool->dma_pages) {
-		WARN(1, "Driver did not DMA map zero-copy buffers");
-		err = -EINVAL;
-		goto err_unreg_xsk;
-	}
 	pool->umem->zc = true;
 	return 0;
 
-err_unreg_xsk:
-	xp_disable_drv_zc(pool);
 err_unreg_pool:
 	if (!force_zc)
 		err = 0; /* fallback to copy mode */
-- 
2.41.0

Re: [PATCH net-next v2 1/3] xsk: Remove non-zero 'dma_page' check in xp_assign_dev

[Paolo Abeni]

On Wed, 2024-02-28 at 19:05 +0800, Yunjian Wang wrote:
> Now dma mappings are used by the physical NICs. However the vNIC
> maybe do not need them. So remove non-zero 'dma_page' check in
> xp_assign_dev.
> 
> Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
> ---
>  net/xdp/xsk_buff_pool.c | 7 -------
>  1 file changed, 7 deletions(-)
> 
> diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
> index ce60ecd48a4d..a5af75b1f43c 100644
> --- a/net/xdp/xsk_buff_pool.c
> +++ b/net/xdp/xsk_buff_pool.c
> @@ -219,16 +219,9 @@ int xp_assign_dev(struct xsk_buff_pool *pool,
>  	if (err)
>  		goto err_unreg_pool;
>  
> -	if (!pool->dma_pages) {
> -		WARN(1, "Driver did not DMA map zero-copy buffers");
> -		err = -EINVAL;
> -		goto err_unreg_xsk;
> -	}

This would unconditionally remove an otherwise valid check for most
NIC. What about let the driver declare it wont need DMA map with a
(pool?) flag.

Cheers,

Paolo

RE: [PATCH net-next v2 1/3] xsk: Remove non-zero 'dma_page' check in xp_assign_dev

[wangyunjian]

> -----Original Message-----
> From: Paolo Abeni [mailto:pabeni@redhat.com]
> Sent: Thursday, February 29, 2024 6:43 PM
> To: wangyunjian <wangyunjian@huawei.com>; mst@redhat.com;
> willemdebruijn.kernel@gmail.com; jasowang@redhat.com; kuba@kernel.org;
> bjorn@kernel.org; magnus.karlsson@intel.com; maciej.fijalkowski@intel.com;
> jonathan.lemon@gmail.com; davem@davemloft.net
> Cc: bpf@vger.kernel.org; netdev@vger.kernel.org;
> linux-kernel@vger.kernel.org; kvm@vger.kernel.org;
> virtualization@lists.linux.dev; xudingke <xudingke@huawei.com>; liwei (DT)
> <liwei395@huawei.com>
> Subject: Re: [PATCH net-next v2 1/3] xsk: Remove non-zero 'dma_page' check in
> xp_assign_dev
> 
> On Wed, 2024-02-28 at 19:05 +0800, Yunjian Wang wrote:
> > Now dma mappings are used by the physical NICs. However the vNIC maybe
> > do not need them. So remove non-zero 'dma_page' check in
> > xp_assign_dev.
> >
> > Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
> > ---
> >  net/xdp/xsk_buff_pool.c | 7 -------
> >  1 file changed, 7 deletions(-)
> >
> > diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c index
> > ce60ecd48a4d..a5af75b1f43c 100644
> > --- a/net/xdp/xsk_buff_pool.c
> > +++ b/net/xdp/xsk_buff_pool.c
> > @@ -219,16 +219,9 @@ int xp_assign_dev(struct xsk_buff_pool *pool,
> >  	if (err)
> >  		goto err_unreg_pool;
> >
> > -	if (!pool->dma_pages) {
> > -		WARN(1, "Driver did not DMA map zero-copy buffers");
> > -		err = -EINVAL;
> > -		goto err_unreg_xsk;
> > -	}
> 
> This would unconditionally remove an otherwise valid check for most NIC. What
> about let the driver declare it wont need DMA map with a
> (pool?) flag.

This check is redundant. The NIC's driver determines whether a DMA map is required.
If the NIC'driver requires the DMA map, it uses the xsk_pool_dma_map function, which
initializes the DMA map and performs a check.

Thanks

> 
> Cheers,
> 
> Paolo

Re: [PATCH net-next v2 1/3] xsk: Remove non-zero 'dma_page' check in xp_assign_dev

[Magnus Karlsson]

On Thu, 29 Feb 2024 at 13:52, wangyunjian <wangyunjian@huawei.com> wrote:
>
> > -----Original Message-----
> > From: Paolo Abeni [mailto:pabeni@redhat.com]
> > Sent: Thursday, February 29, 2024 6:43 PM
> > To: wangyunjian <wangyunjian@huawei.com>; mst@redhat.com;
> > willemdebruijn.kernel@gmail.com; jasowang@redhat.com; kuba@kernel.org;
> > bjorn@kernel.org; magnus.karlsson@intel.com; maciej.fijalkowski@intel.com;
> > jonathan.lemon@gmail.com; davem@davemloft.net
> > Cc: bpf@vger.kernel.org; netdev@vger.kernel.org;
> > linux-kernel@vger.kernel.org; kvm@vger.kernel.org;
> > virtualization@lists.linux.dev; xudingke <xudingke@huawei.com>; liwei (DT)
> > <liwei395@huawei.com>
> > Subject: Re: [PATCH net-next v2 1/3] xsk: Remove non-zero 'dma_page' check in
> > xp_assign_dev
> >
> > On Wed, 2024-02-28 at 19:05 +0800, Yunjian Wang wrote:
> > > Now dma mappings are used by the physical NICs. However the vNIC maybe
> > > do not need them. So remove non-zero 'dma_page' check in
> > > xp_assign_dev.
> > >
> > > Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
> > > ---
> > >  net/xdp/xsk_buff_pool.c | 7 -------
> > >  1 file changed, 7 deletions(-)
> > >
> > > diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c index
> > > ce60ecd48a4d..a5af75b1f43c 100644
> > > --- a/net/xdp/xsk_buff_pool.c
> > > +++ b/net/xdp/xsk_buff_pool.c
> > > @@ -219,16 +219,9 @@ int xp_assign_dev(struct xsk_buff_pool *pool,
> > >     if (err)
> > >             goto err_unreg_pool;
> > >
> > > -   if (!pool->dma_pages) {
> > > -           WARN(1, "Driver did not DMA map zero-copy buffers");
> > > -           err = -EINVAL;
> > > -           goto err_unreg_xsk;
> > > -   }
> >
> > This would unconditionally remove an otherwise valid check for most NIC. What
> > about let the driver declare it wont need DMA map with a
> > (pool?) flag.
>
> This check is redundant. The NIC's driver determines whether a DMA map is required.
> If the NIC'driver requires the DMA map, it uses the xsk_pool_dma_map function, which
> initializes the DMA map and performs a check.

Just to provide some context: I put this check there many years ago to
guard against a zero-copy driver writer forgetting to call
xsk_pool_dma_map() during the implementation phase. A working driver
will always have pool->dma_pages != NULL. If you both think that this
check is too much of a precaution, then I have no problem getting rid
of it. Just thought that a text warning would be nicer than a crash
later.

Thanks: Magnus

> Thanks
>
> >
> > Cheers,
> >
> > Paolo
>