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

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Date: Sat, 01 Jul 2023 15:51:30 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.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

Maciej Fijalkowski <maciej.fijalkowski@...el.com> writes:

> 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?

Yes, the latter. I have no objection to the internal field being set to
1 by default or anything, I just think we should omit the netlink
attribute when it doesn't have a meaningful value, to avoid confusion -
being able to do that is one of the nice properties of netlink, after all :)

-Toke