netdev - Re: [PATCH v4 bpf-next 15/22] xsk: add multi-buffer documentation

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Date: Fri, 30 Jun 2023 20:00:12 +0200
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>
CC: Magnus Karlsson <magnus.karlsson@...il.com>, Jakub Kicinski
	<kuba@...nel.org>, <bpf@...r.kernel.org>, <ast@...nel.org>,
	<daniel@...earbox.net>, <andrii@...nel.org>, <netdev@...r.kernel.org>,
	<magnus.karlsson@...el.com>, <bjorn@...nel.org>,
	<tirthendu.sarkar@...el.com>, <simon.horman@...igine.com>
Subject: Re: [PATCH v4 bpf-next 15/22] xsk: add multi-buffer documentation

On Thu, Jun 29, 2023 at 10:57:05PM +0200, Toke Høiland-Jørgensen wrote:
> Maciej Fijalkowski <maciej.fijalkowski@...el.com> writes:
> 
> > On Wed, Jun 28, 2023 at 11:02:06PM +0200, Toke Høiland-Jørgensen wrote:
> >> > diff --git a/net/core/netdev-genl.c b/net/core/netdev-genl.c
> >> > index a4270fafdf11..b24244f768e3 100644
> >> > --- a/net/core/netdev-genl.c
> >> > +++ b/net/core/netdev-genl.c
> >> > @@ -19,6 +19,8 @@ netdev_nl_dev_fill(struct net_device *netdev, struct sk_buff *rsp,
> >> >  		return -EMSGSIZE;
> >> >  
> >> >  	if (nla_put_u32(rsp, NETDEV_A_DEV_IFINDEX, netdev->ifindex) ||
> >> > +	    nla_put_u32(rsp, NETDEV_A_DEV_XDP_ZC_MAX_SEGS,
> >> > +			netdev->xdp_zc_max_segs) ||
> >> 
> >> Should this be omitted if the driver doesn't support zero-copy at all?
> >
> > This is now set independently when allocing net_device struct, so this can
> > be read without issues. Furthermore this value should not be used to find
> > out if underlying driver supports ZC or not - let us keep using
> > xdp_features for that.
> >
> > Does it make sense?
> 
> Yes, I agree we shouldn't use this field for that. However, I am not
> sure I trust all userspace applications to get that right, so I fear
> some will end up looking at the field even when the flag is not set,
> which will lead to confused users. So why not just omit the property
> entirely when the flag is not set? :)

I think that if you would read anything different than default 1 from this
field and your driver does not zupport even ZC then your driver is wrong.
It's like reporting something via xdp_features and not supporting it. You
only overwrite this within your driver *if* you support ZC multi-buffer.

OTOH were you referring to omitting putting the u32 to netlink response at
all?

> 
> -Toke
>