Re: [dpdk-stable] [PATCH] net/af_xdp: fix 32-bit build for older kernels

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 11/12/2020 4:35 PM, Ciara Loftus wrote:
> 'uint64_t' is used to hold pointers in multiple locations in the
> copy-mode code (used for kernels before 5.4). For a 32-bit build
> this assumption is wrong and results in build errors. This commit
> replaces such instances of 'uint64_t' with 'uintptr_t'.
> 
> While the copy-mode code will now compile for 32-bit, the PMD is
> not expected to work and will fail at initialisation due to some
> limitations in the kernel that were subsequently removed in v5.4.
> Add a note to the docs to flag this limitation.
> 
> Fixes: f1debd77efaf ("net/af_xdp: introduce AF_XDP PMD")
> Fixes: d8a210774e1d ("net/af_xdp: support unaligned umem chunks")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
> ---
>   doc/guides/nics/af_xdp.rst          | 1 +
>   drivers/net/af_xdp/rte_eth_af_xdp.c | 6 +++---
>   2 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/doc/guides/nics/af_xdp.rst b/doc/guides/nics/af_xdp.rst
> index 052e59a3ae..5ed24374f8 100644
> --- a/doc/guides/nics/af_xdp.rst
> +++ b/doc/guides/nics/af_xdp.rst
> @@ -50,6 +50,7 @@ This is a Linux-specific PMD, thus the following prerequisites apply:
>   *  For PMD zero copy, it requires kernel version later than v5.4-rc1;
>   *  For shared_umem, it requires kernel version v5.10 or later and libbpf version
>      v0.2.0 or later.
> +*  For 32-bit OS, a kernel with version 5.4 or later is required.
>   

+1 to doc update

>   Set up an af_xdp interface
>   -----------------------------
> diff --git a/drivers/net/af_xdp/rte_eth_af_xdp.c b/drivers/net/af_xdp/rte_eth_af_xdp.c
> index 4076ff797c..75ff1c00b2 100644
> --- a/drivers/net/af_xdp/rte_eth_af_xdp.c
> +++ b/drivers/net/af_xdp/rte_eth_af_xdp.c
> @@ -349,7 +349,7 @@ af_xdp_rx_cp(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
>   
>   	for (i = 0; i < rcvd; i++) {
>   		const struct xdp_desc *desc;
> -		uint64_t addr;
> +		uintptr_t addr;
>   		uint32_t len;
>   		void *pkt;
>   
> @@ -402,7 +402,7 @@ pull_umem_cq(struct xsk_umem_info *umem, int size, struct xsk_ring_cons *cq)
>   	n = xsk_ring_cons__peek(cq, size, &idx_cq);
>   
>   	for (i = 0; i < n; i++) {
> -		uint64_t addr;
> +		uintptr_t addr;
>   		addr = *xsk_ring_cons__comp_addr(cq, idx_cq++);

Hi Ciara,

As far as I can see the API 'xsk_ring_cons__comp_addr()' returns fixed size 
variable ('__u64'),
and when the PMD is compiled for 32bit, won't it be assigning a 64bit variable 
to the 32bit storage.

I guess libbpf also needs to be adjusted for the 32bit support, what about 
making PMD changes after libbpf changed?

>   #if defined(XDP_UMEM_UNALIGNED_CHUNK_FLAG)
>   		addr = xsk_umem__extract_addr(addr);
> @@ -1005,7 +1005,7 @@ xsk_umem_info *xdp_umem_configure(struct pmd_internals *internals,
>   	char ring_name[RTE_RING_NAMESIZE];
>   	char mz_name[RTE_MEMZONE_NAMESIZE];
>   	int ret;
> -	uint64_t i;
> +	uintptr_t i;
>   

Not sure on this one, 'i' seems not to hold a pointer but index, and result of 
calculation cast to "void *", I assume intention is to prevent calculation 
result to be 64 bit to cover the case "void *" is 4 bytes, for that what do you 
think making variable uint32_t?