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.
