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

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

On 11/16/2020 2:24 PM, Loftus, Ciara wrote:
>>
>> 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.
> 
> Correct. However we can assume the higher 32bits are zero in this case.
> The 'addr' we are consuming via this API will be one which we previously enqueued to the buf_ring and we always cast to (void *) on enqueue.
> 

My concern was if compiler complains about this, since from compiler perspective 
there is a 64 -> 32 bit variable assignment which can lead data loss.

>>
>> I guess libbpf also needs to be adjusted for the 32bit support, what about
>> making PMD changes after libbpf changed?
> 
> I'm not sure whether this is planned but maybe it makes sense to wait and see rather than relying on assumptions above.
> 
>>
>>>    #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?
> 
> Do you suggest something like:
> #ifdef RTE_ARCH_64
>         uint64_t i;
> #else
>         uint32_t i;
> #endif

I was thinking only use 32 bit, since 32 -> 64 bit assignment will not cause any 
problem.

> 
> I can submit a v2 with just the doc update and hold off on the other changes until the necessary changes to libbpf are in place. Let me know what you think.
> 

+1 to this, after 32 bit libbpf is ready, it can be easier to do changes to the PMD.