Re: [PATCH RFC bpf-next 07/20] xdp: Track if metadata is supported in xdp_frame <> xdp_buff conversions

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 05/03/2025 18.02, Arthur Fabre wrote:
> On Wed Mar 5, 2025 at 4:24 PM CET, Alexander Lobakin wrote:
>> From: Arthur <arthur@arthurfabre.com>
>> Date: Wed, 05 Mar 2025 15:32:04 +0100
>>
>>> From: Arthur Fabre <afabre@cloudflare.com>
>>>
>>> xdp_buff stores whether metadata is supported by a NIC by setting
>>> data_meta to be greater than data.
>>>
>>> But xdp_frame only stores the metadata size (as metasize), so converting
>>> between xdp_frame and xdp_buff is lossy.
>>>
>>> Steal an unused bit in xdp_frame to track whether metadata is supported
>>> or not.
>>>
>>> This will lets us have "generic" functions for setting skb fields from
>>> either xdp_frame or xdp_buff from drivers.
>>>
>>> Signed-off-by: Arthur Fabre <afabre@cloudflare.com>
>>> ---
>>>   include/net/xdp.h | 10 +++++++++-
>>>   1 file changed, 9 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/net/xdp.h b/include/net/xdp.h
>>> index 58019fa299b56dbd45c104fdfa807f73af6e4fa4..84afe07d09efdb2ab0cb78b904f02cb74f9a56b6 100644
>>> --- a/include/net/xdp.h
>>> +++ b/include/net/xdp.h
>>> @@ -116,6 +116,9 @@ static __always_inline void xdp_buff_set_frag_pfmemalloc(struct xdp_buff *xdp)
>>>   	xdp->flags |= XDP_FLAGS_FRAGS_PF_MEMALLOC;
>>>   }
>>>   
>>> +static bool xdp_data_meta_unsupported(const struct xdp_buff *xdp);
>>> +static void xdp_set_data_meta_invalid(struct xdp_buff *xdp);
>>> +
>>>   static __always_inline void *xdp_buff_traits(const struct xdp_buff *xdp)
>>>   {
>>>   	return xdp->data_hard_start + _XDP_FRAME_SIZE;
>>> @@ -270,7 +273,9 @@ struct xdp_frame {
>>>   	void *data;
>>>   	u32 len;
>>>   	u32 headroom;
>>> -	u32 metasize; /* uses lower 8-bits */
>>> +	u32	:23, /* unused */
>>> +		meta_unsupported:1,
>>> +		metasize:8;
>>
>> See the history of this structure how we got rid of using bitfields here
>> and why.
>>
>> ...because of performance.
>>
>> Even though metasize uses only 8 bits, 1-byte access is slower than
>> 32-byte access.
> 
> Interesting, thanks!
> 

I agree with Olek, we should not use bitfields.  Thanks for catching this.

(The xdp_frame have a flags member...)
Why don't we use the flags member for storing this information?


>> I was going to write "you can use the fact that metasize is always a
>> multiple of 4 or that it's never > 252, for example, you could reuse LSB
>> as a flag indicating that meta is not supported", but first of all
>>
>> Do we still have drivers which don't support metadata?
>> Why don't they do that? It's not HW-specific or even driver-specific.
>> They don't reserve headroom? Then they're invalid, at least XDP_REDIRECT
>> won't work.
>>

I'm fairly sure that all drivers support XDP_REDIRECT.
Except didn't Lorenzo add a feature bit for this?
(so, some drivers might explicitly not-support this)

>> So maybe we need to fix those drivers first, if there are any.
> 
> Most drivers don't support metadata unfortunately:
> 
>> rg -U "xdp_prepare_buff\([^)]*false\);" drivers/net/
> drivers/net/tun.c
> 1712:		xdp_prepare_buff(&xdp, buf, pad, len, false);
> 
> drivers/net/ethernet/microsoft/mana/mana_bpf.c
> 94:	xdp_prepare_buff(xdp, buf_va, XDP_PACKET_HEADROOM, pkt_len, false);
> 
> drivers/net/ethernet/marvell/mvneta.c
> 2344:	xdp_prepare_buff(xdp, data, pp->rx_offset_correction + MVNETA_MH_SIZE,
> 2345:			 data_len, false);
> 
> drivers/net/ethernet/marvell/octeontx2/nic/otx2_txrx.c
> 1436:	xdp_prepare_buff(&xdp, hard_start, OTX2_HEAD_ROOM,
> 1437:			 cqe->sg.seg_size, false);
> 
> drivers/net/ethernet/socionext/netsec.c
> 1021:		xdp_prepare_buff(&xdp, desc->addr, NETSEC_RXBUF_HEADROOM,
> 1022:				 pkt_len, false);
> 
> drivers/net/ethernet/google/gve/gve_rx.c
> 740:	xdp_prepare_buff(&new, frame, headroom, len, false);
> 859:		xdp_prepare_buff(&xdp, page_info->page_address +
> 860:				 page_info->page_offset, GVE_RX_PAD,
> 861:				 len, false);
> 
> drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
> 3984:			xdp_prepare_buff(&xdp, data,
> 3985:					 MVPP2_MH_SIZE + MVPP2_SKB_HEADROOM,
> 3986:					 rx_bytes, false);
> 
> drivers/net/ethernet/aquantia/atlantic/aq_ring.c
> 794:		xdp_prepare_buff(&xdp, hard_start, rx_ring->page_offset,
> 795:				 buff->len, false);
> 
> drivers/net/ethernet/cavium/thunder/nicvf_main.c
> 554:	xdp_prepare_buff(&xdp, hard_start, data - hard_start, len, false);
> 
> drivers/net/ethernet/ti/cpsw_new.c
> 348:		xdp_prepare_buff(&xdp, pa, headroom, size, false);
> 
> drivers/net/ethernet/freescale/enetc/enetc.c
> 1710:	xdp_prepare_buff(xdp_buff, hard_start - rx_ring->buffer_offset,
> 1711:			 rx_ring->buffer_offset, size, false);
> 
> drivers/net/ethernet/ti/am65-cpsw-nuss.c
> 1335:		xdp_prepare_buff(&xdp, page_addr, AM65_CPSW_HEADROOM,
> 1336:				 pkt_len, false);
> 
> drivers/net/ethernet/ti/cpsw.c
> 403:		xdp_prepare_buff(&xdp, pa, headroom, size, false);
> 
> drivers/net/ethernet/sfc/rx.c
> 289:	xdp_prepare_buff(&xdp, *ehp - EFX_XDP_HEADROOM, EFX_XDP_HEADROOM,
> 290:			 rx_buf->len, false);
> 
> drivers/net/ethernet/mediatek/mtk_eth_soc.c
> 2097:			xdp_prepare_buff(&xdp, data, MTK_PP_HEADROOM, pktlen,
> 2098:					 false);
> 
> drivers/net/ethernet/sfc/siena/rx.c
> 291:	xdp_prepare_buff(&xdp, *ehp - EFX_XDP_HEADROOM, EFX_XDP_HEADROOM,
> 292:			 rx_buf->len, false)
> 
> I don't know if it's just because no one has added calls to
> skb_metadata_set() in yet, or if there's a more fundamental reason.
> 

I simply think driver developers have been lazy.

If someone want some easy kernel commits, these drivers should be fairly
easy to fix...

> I think they all reserve some amount of headroom, but not always the
> full XDP_PACKET_HEADROOM. Eg sfc:
> 

The Intel drivers use 192 (AFAIK if that is still true). The API ended
up supporting non-standard XDP_PACKET_HEADROOM, due to the Intel
drivers, when XDP support was added to those (which is a long time ago now).

> drivers/net/ethernet/sfc/net_driver.h:
> /* Non-standard XDP_PACKET_HEADROOM and tailroom to satisfy XDP_REDIRECT and
>   * still fit two standard MTU size packets into a single 4K page.
>   */
> #define EFX_XDP_HEADROOM	128
> 

This is smaller than most drivers, but still have enough headroom for 
xdp_frame + traits.

> If it's just because skb_metadata_set() is missing, I can take the
> patches from this series that adds a "generic" XDP -> skb hook ("trait:
> Propagate presence of traits to sk_buff"), have it call
> skb_metadata_set(), and try to add it to all the drivers in a separate
> series.
> 

I think someone should cleanup those drivers and add support.

--Jesper

>>>   	/* Lifetime of xdp_rxq_info is limited to NAPI/enqueue time,
>>>   	 * while mem_type is valid on remote CPU.
>>>   	 */
>>> @@ -369,6 +374,8 @@ void xdp_convert_frame_to_buff(const struct xdp_frame *frame,
>>>   	xdp->data = frame->data;
>>>   	xdp->data_end = frame->data + frame->len;
>>>   	xdp->data_meta = frame->data - frame->metasize;
>>> +	if (frame->meta_unsupported)
>>> +		xdp_set_data_meta_invalid(xdp);
>>>   	xdp->frame_sz = frame->frame_sz;
>>>   	xdp->flags = frame->flags;
>>>   }
>>> @@ -396,6 +403,7 @@ int xdp_update_frame_from_buff(const struct xdp_buff *xdp,
>>>   	xdp_frame->len  = xdp->data_end - xdp->data;
>>>   	xdp_frame->headroom = headroom - sizeof(*xdp_frame);
>>>   	xdp_frame->metasize = metasize;
>>> +	xdp_frame->meta_unsupported = xdp_data_meta_unsupported(xdp);
>>>   	xdp_frame->frame_sz = xdp->frame_sz;
>>>   	xdp_frame->flags = xdp->flags;
>>
>> Thanks,
>> Olek