Received-SPF: pass (google.com: domain of linux-kernel+bounces-90696-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Precedence: bulk
MIME-Version: 1.0
References: <1709118325-120336-1-git-send-email-wangyunjian@huawei.com>
 <75b6f7686c03519a1aaeb461618070747890143b.camel@redhat.com> <55ef319de7084614b1883018f69de1eb@huawei.com>
In-Reply-To: <55ef319de7084614b1883018f69de1eb@huawei.com>
From: Magnus Karlsson <magnus.karlsson@gmail.com>
Date: Mon, 4 Mar 2024 14:58:11 +0100
Message-ID: <CAJ8uoz0xjdw7iHjP9=9wFPq21A6LdcYuGZBgwUKRemzkB5XoNg@mail.gmail.com>
Subject: Re: [PATCH net-next v2 1/3] xsk: Remove non-zero 'dma_page' check in xp_assign_dev
To: wangyunjian <wangyunjian@huawei.com>
Cc: Paolo Abeni <pabeni@redhat.com>, "mst@redhat.com" <mst@redhat.com>, 
	"willemdebruijn.kernel@gmail.com" <willemdebruijn.kernel@gmail.com>, 
	"jasowang@redhat.com" <jasowang@redhat.com>, "kuba@kernel.org" <kuba@kernel.org>, 
	"bjorn@kernel.org" <bjorn@kernel.org>, "magnus.karlsson@intel.com" <magnus.karlsson@intel.com>, 
	"maciej.fijalkowski@intel.com" <maciej.fijalkowski@intel.com>, 
	"jonathan.lemon@gmail.com" <jonathan.lemon@gmail.com>, "davem@davemloft.net" <davem@davemloft.net>, 
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>, "netdev@vger.kernel.org" <netdev@vger.kernel.org>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>, 
	"virtualization@lists.linux.dev" <virtualization@lists.linux.dev>, xudingke <xudingke@huawei.com>, 
	"liwei (DT)" <liwei395@huawei.com>
Content-Type: text/plain; charset="UTF-8"

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
>