RE: [PATCH 1/2] lib/ipsec: compile ipsec on Windows

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> Removed VLA for compatibility with MSVC (which does not support VLAs).
> Used alloca when a constant fixed length that can be used instead is
> not known.
> 
...
 
> Signed-off-by: Andre Muezerie <andremue@linux.microsoft.com>
> ---

...

> diff --git a/lib/ipsec/esp_inb.c b/lib/ipsec/esp_inb.c
> index f159bf7460..305ac48dc5 100644
> --- a/lib/ipsec/esp_inb.c
> +++ b/lib/ipsec/esp_inb.c
> @@ -370,8 +370,9 @@ esp_inb_pkt_prepare(const struct rte_ipsec_session *ss, struct rte_mbuf *mb[],
>  	struct rte_cryptodev_sym_session *cs;
>  	struct replay_sqn *rsn;
>  	union sym_op_data icv;
> -	uint32_t dr[num];
> +	uint32_t *dr;

I understand the intention, but obviously as the lib maintainer, I am no
very happy with mechanic replacement of VLAs with alloca() calls.
Again, while it helps to make MSVC happy, it doesn't really solve a potential problem
with using VLAs here - I we can end-up with stack overflowed if someone will call
one of these functions with  input arrays of huge size.
It would be much better to re-work the internal functions to work over fixed-size array
an then call them in a loop. 
The main issue here is that by API convention we need to re-arrange input array by
moving bad (not processed) mbuf after the good ones.
But even with that splitting processing into multiple iterations over small chunks
might help I think.  

> +	dr = alloca(sizeof(uint32_t) * num);
>  	sa = ss->sa;
>  	cs = ss->crypto.ses;
>  	rsn = rsn_acquire(sa);
> @@ -576,12 +577,16 @@ tun_process(struct rte_ipsec_sa *sa, struct rte_mbuf *mb[],
>  	    uint32_t sqn[], uint32_t dr[], uint16_t num, uint8_t sqh_len)
>  {
>  	uint32_t adj, i, k, tl, bytes;
> -	uint32_t hl[num], to[num];
> -	struct rte_esp_tail espt[num];
> -	struct rte_mbuf *ml[num];
> +	uint32_t *hl, *to;
> +	struct rte_esp_tail *espt;
> +	struct rte_mbuf **ml;
>  	const void *outh;
>  	void *inh;
> 
> +	hl = alloca(sizeof(uint32_t) * num);
> +	to = alloca(sizeof(uint32_t) * num);
> +	espt = alloca(sizeof(struct rte_esp_tail) * num);
> +	ml = alloca(sizeof(struct rte_mbuf *) * num);
>  	/*
>  	 * remove icv, esp trailer and high-order
>  	 * 32 bits of esn from packet length