On 19/05/2023 18.35, Alexander Lobakin wrote:
From: Jesper Dangaard Brouer <jbrouer@xxxxxxxxxx>
Date: Tue, 16 May 2023 17:35:27 +0200
On 16/05/2023 14.37, Alexander Lobakin wrote:
From: Larysa Zaremba<larysa.zaremba@xxxxxxxxx>
Date: Mon, 15 May 2023 19:08:39 +0200
On Mon, May 15, 2023 at 06:17:02PM +0200, Jesper Dangaard Brouer wrote:
On 12/05/2023 17.26, Larysa Zaremba wrote:
From: Aleksander Lobakin<aleksander.lobakin@xxxxxxxxx>
When using XDP hints, metadata sometimes has to be much bigger
than 32 bytes. Relax the restriction, allow metadata larger than 32
bytes
and make __skb_metadata_differs() work with bigger lengths.
Now size of metadata is only limited by the fact it is stored as u8
in skb_shared_info, so maximum possible value is 255.
I'm confused, IIRC the metadata area isn't stored "in skb_shared_info".
The maximum possible size is limited by the XDP headroom, which is also
shared/limited with/by xdp_frame. I must be reading the sentence
wrong,
somehow.
skb_shared_info::meta_size is u8. Since metadata gets carried from
xdp_buff to skb, this check is needed (it's compile-time constant
anyway).
Check for headroom is done separately already (two sentences below).
Damn, argh, for SKBs the "meta_len" is stored in skb_shared_info, which
is located on another cacheline.
That is a sure way to KILL performance! :-(
Have you read the code? I use type_max(typeof_member(shinfo, meta_len)),
what performance are you talking about?
Not talking about your changes (in this patch).
I'm realizing that SKBs using metadata area will have a performance hit
due to accessing another cacheline (the meta_len in skb_shared_info).
IIRC Daniel complained about this performance hit (in the past), I guess
this explains it. IIRC Cilium changed to use percpu variables/datastore
to workaround this.
The whole xdp_metalen_invalid() gets expanded into:
return (metalen % 4) || metalen > 255;
at compile-time. All those typeof shenanigans are only to not open-code
meta_len's type/size/max.
But only use for SKBs that gets created from xdp with metadata, right?
Normal netstack processing actually access this skb_shinfo->meta_len in
gro_list_prepare(). As the caller dev_gro_receive() later access other
memory in skb_shared_info, then the GRO code path already takes this hit
to begin with.
--Jesper
