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

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Date: Mon, 4 Mar 2024 14:58:11 +0100
From: Magnus Karlsson <magnus.karlsson@...il.com>
To: wangyunjian <wangyunjian@...wei.com>
Cc: Paolo Abeni <pabeni@...hat.com>, "mst@...hat.com" <mst@...hat.com>, 
	"willemdebruijn.kernel@...il.com" <willemdebruijn.kernel@...il.com>, 
	"jasowang@...hat.com" <jasowang@...hat.com>, "kuba@...nel.org" <kuba@...nel.org>, 
	"bjorn@...nel.org" <bjorn@...nel.org>, "magnus.karlsson@...el.com" <magnus.karlsson@...el.com>, 
	"maciej.fijalkowski@...el.com" <maciej.fijalkowski@...el.com>, 
	"jonathan.lemon@...il.com" <jonathan.lemon@...il.com>, "davem@...emloft.net" <davem@...emloft.net>, 
	"bpf@...r.kernel.org" <bpf@...r.kernel.org>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"virtualization@...ts.linux.dev" <virtualization@...ts.linux.dev>, xudingke <xudingke@...wei.com>, 
	"liwei (DT)" <liwei395@...wei.com>
Subject: Re: [PATCH net-next v2 1/3] xsk: Remove non-zero 'dma_page' check in xp_assign_dev

On Thu, 29 Feb 2024 at 13:52, wangyunjian <wangyunjian@...wei.com> wrote:
>
> > -----Original Message-----
> > From: Paolo Abeni [mailto:pabeni@...hat.com]
> > Sent: Thursday, February 29, 2024 6:43 PM
> > To: wangyunjian <wangyunjian@...wei.com>; mst@...hat.com;
> > willemdebruijn.kernel@...il.com; jasowang@...hat.com; kuba@...nel.org;
> > bjorn@...nel.org; magnus.karlsson@...el.com; maciej.fijalkowski@...el.com;
> > jonathan.lemon@...il.com; davem@...emloft.net
> > Cc: bpf@...r.kernel.org; netdev@...r.kernel.org;
> > linux-kernel@...r.kernel.org; kvm@...r.kernel.org;
> > virtualization@...ts.linux.dev; xudingke <xudingke@...wei.com>; liwei (DT)
> > <liwei395@...wei.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@...wei.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
>