Hi Stephen,

On Sat, 11 Nov 2023, Stephen Hemminger wrote:

> While examining the use of VLA in DPDK, ran into a bug in sfc driver.
>
> If DPDK is built with -Wvla, then the RTE_BUILD_BUG_ON() macro won't work
> as written. Experimenting with a better more portable version of that macro
> as:
> 	#define RTE_BUILD_BUG_ON(e) _Static_assert(!(e),  #e)

First of all, thanks for the effort. Very helpful.
Please see below.

>
> revealed that the SFC driver was calling RTE_BUILD_BUG_ON with non constant
> expression.
>
> ../drivers/net/sfc/sfc_ef100_tx.c: In function ‘sfc_ef100_tx_pkt_descs_max’:
> ../lib/eal/include/rte_common.h:585:20: warning: comparison of integer expressions of different signedness: ‘unsigned int’ and ‘int’ [-Wsign-compare]
>  585 |                 _a < _b ? _a : _b; \
>      |                    ^
> ../lib/eal/include/rte_common.h:498:46: note: in definition of macro ‘RTE_BUILD_BUG_ON’
>  498 | #define RTE_BUILD_BUG_ON(e) _Static_assert(!(e),  #e)
>      |                                              ^
> ../drivers/net/sfc/sfc_ef100_tx.c:566:34: note: in expansion of macro ‘RTE_MIN’
>  566 |                                  RTE_MIN((unsigned int)EFX_MAC_PDU_MAX,
>      |                                  ^~~~~~~
> ../lib/eal/include/rte_common.h:585:32: warning: operand of ‘?:’ changes signedness from ‘int’ to ‘unsigned int’ due to unsignedness of other operand [-Wsign-compare]
>  585 |                 _a < _b ? _a : _b; \
>      |                                ^~
> ../lib/eal/include/rte_common.h:498:46: note: in definition of macro ‘RTE_BUILD_BUG_ON’
>  498 | #define RTE_BUILD_BUG_ON(e) _Static_assert(!(e),  #e)
>      |                                              ^
> ../drivers/net/sfc/sfc_ef100_tx.c:566:34: note: in expansion of macro ‘RTE_MIN’
>  566 |                                  RTE_MIN((unsigned int)EFX_MAC_PDU_MAX,
>      |                                  ^~~~~~~
> ../lib/eal/include/rte_common.h:498:44: error: expression in static assertion is not constant
>  498 | #define RTE_BUILD_BUG_ON(e) _Static_assert(!(e),  #e)
>      |                                            ^~~~
> ../drivers/net/sfc/sfc_ef100_tx.c:565:17: note: in expansion of macro ‘RTE_BUILD_BUG_ON’
>
>
> The problem is that Gcc does not evaluate a ternary operator expression
> with all constants at compile time to produce a constant value! Apparently,
> the language standards leave this as ambiguous.
>
> If the code is expanded into two assertions as:
>
> diff --git a/drivers/net/sfc/sfc_ef100_tx.c b/drivers/net/sfc/sfc_ef100_tx.c
> index 1b6374775f07..25e6633d6679 100644
> --- a/drivers/net/sfc/sfc_ef100_tx.c
> +++ b/drivers/net/sfc/sfc_ef100_tx.c
> @@ -562,9 +562,8 @@ sfc_ef100_tx_pkt_descs_max(const struct rte_mbuf *m)
>                 * Make sure that the first segment does not need fragmentation
>                 * (split into many Tx descriptors).
>                 */
> -               RTE_BUILD_BUG_ON(SFC_EF100_TX_SEND_DESC_LEN_MAX <
> -                                RTE_MIN((unsigned int)EFX_MAC_PDU_MAX,
> -                                SFC_MBUF_SEG_LEN_MAX));
> +               RTE_BUILD_BUG_ON(SFC_EF100_TX_SEND_DESC_LEN_MAX < EFX_MAC_PDU_MAX);
> +               RTE_BUILD_BUG_ON(SFC_EF100_TX_SEND_DESC_LEN_MAX < SFC_MBUF_SEG_LEN_MAX);
>        }
>
>        if (m->ol_flags & sfc_dp_mport_override) {
>
> Then a new problem arises:
> In file included from ../lib/mbuf/rte_mbuf.h:36,
>                 from ../drivers/net/sfc/sfc_ef100_tx.c:12:
> ../drivers/net/sfc/sfc_ef100_tx.c: In function ‘sfc_ef100_tx_pkt_descs_max’:
> ../lib/eal/include/rte_common.h:498:29: error: static assertion failed: "SFC_EF100_TX_SEND_DESC_LEN_MAX < SFC_MBUF_SEG_LEN_MAX"
>  498 | #define RTE_BUILD_BUG_ON(e) _Static_assert(!(e),  #e)
>      |                             ^~~~~~~~~~~~~~
> ../drivers/net/sfc/sfc_ef100_tx.c:566:17: note: in expansion of macro ‘RTE_BUILD_BUG_ON’
>  566 |                 RTE_BUILD_BUG_ON(SFC_EF100_TX_SEND_DESC_LEN_MAX < SFC_MBUF_SEG_LEN_MAX);
>      |                 ^~~~~~~~~~~~~~~~
>
> Building a little program to unwind the #defines reveals:
>
> SFC_EF100_TX_SEND_DESC_LEN_MAX = 16383
> EFX_MAC_PDU_MAX = 9240
> SFC_MBUF_SEG_LEN_MAX = 65535
>
> I.e:
> 	RTE_BUILD_BUG_ON(16383 < RTE_MIN(9240, 65535));
>
>
> Therefore the current driver should be getting build bug, but the existing macro
> hides it.

As far as I understand, the intention behind this check is to make sure
that SFC_EF100_TX_SEND_DESC_LEN_MAX represents enough room to
accommodate either EFX_MAC_PDU_MAX or SFC_MBUF_SEG_LEN_MAX bytes,
whichever is smaller. Is 16383 sufficient to accommodate 9240?
I think so. Do you agree?

That being said, indeed, applying the "more portable version" of yours
results in me seeing the warning about a non-constant expression.

Applying the following patch makes all errors disappear
when building with either version of RTE_BUILD_BUG_ON:

diff --git a/drivers/net/sfc/sfc_ef100_tx.c b/drivers/net/sfc/sfc_ef100_tx.c
index 1b6374775f..01f37c2616 100644
--- a/drivers/net/sfc/sfc_ef100_tx.c
+++ b/drivers/net/sfc/sfc_ef100_tx.c
@@ -563,7 +563,7 @@ sfc_ef100_tx_pkt_descs_max(const struct rte_mbuf *m)
                  * (split into many Tx descriptors).
                  */
                 RTE_BUILD_BUG_ON(SFC_EF100_TX_SEND_DESC_LEN_MAX <
-                                RTE_MIN((unsigned int)EFX_MAC_PDU_MAX,
+                                MIN((unsigned int)EFX_MAC_PDU_MAX,
                                  SFC_MBUF_SEG_LEN_MAX));
         }

with MIN being defined in drivers/common/sfc_efx/efsys.h as
#define MIN(v1, v2)	((v1) < (v2) ? (v1) : (v2))

Would that be an acceptable fix? Or am I missing something?

Thank you.