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-----邮件原件-----
发件人: dev [off-list ref] 代表 Ananyev, Konstantin
发送时间: Tuesday, October 26, 2021 8:57 PM
收件人: Feifei Wang [off-list ref]
抄送: dev@dpdk.org; nd [off-list ref]; Ruifeng Wang
[off-list ref]; nd [off-list ref]
主题: Re: [dpdk-dev] [PATCH v5 4/5] lib/bpf: use wait event scheme for Rx/Tx
iteration

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Hi Feifei,

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Instead of polling for cbi->use to be updated, use wait event scheme.

Furthermore, delete 'const' for 'bpf_eth_cbi_wait'. This is
because of a compilation error:
------------------------------------------------------------------
-----
../lib/eal/include/rte_common.h:36:13: error: read-only variable ‘value’
used as ‘asm’ output
   36 | #define asm __asm__
      |             ^~~~~~~

../lib/eal/arm/include/rte_pause_64.h:66:3: note: in expansion of

macro ‘asm’

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   66 |   asm volatile("ldaxr %w[tmp], [%x[addr]]" \
      |   ^~~

../lib/eal/arm/include/rte_pause_64.h:96:3: note: in expansion of
macro ‘__LOAD_EXC_32’
   96 |   __LOAD_EXC_32((src), dst, memorder)     \
      |   ^~~~~~~~~~~~~

../lib/eal/arm/include/rte_pause_64.h:167:4: note: in expansion of
macro ‘__LOAD_EXC’
  167 |    __LOAD_EXC((addr), value, memorder, size) \
      |    ^~~~~~~~~~

../lib/bpf/bpf_pkt.c:125:3: note: in expansion of macro ‘rte_wait_event’
  125 |   rte_wait_event(&cbi->use, UINT32_MAX, ==, puse,
------------------------------------------------------------------
-----

Signed-off-by: Feifei Wang <redacted>
Reviewed-by: Ruifeng Wang <redacted>
---
 lib/bpf/bpf_pkt.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/lib/bpf/bpf_pkt.c b/lib/bpf/bpf_pkt.c index

6e8248f0d6..213d44a75a 100644

--- a/lib/bpf/bpf_pkt.c
+++ b/lib/bpf/bpf_pkt.c

@@ -111,9 +111,9 @@ bpf_eth_cbi_unuse(struct bpf_eth_cbi *cbi)
  * Waits till datapath finished using given callback.
  */
 static void
-bpf_eth_cbi_wait(const struct bpf_eth_cbi *cbi)
+bpf_eth_cbi_wait(struct bpf_eth_cbi *cbi)

Hi, Konstantin

For this bpf patch, I delete 'const' through this is contrary to
what we discussed earlier. This is because if  we keep 'constant' here and

use 'rte_wait_event'

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new macro, compiler will report error. And earlier the arm version
cannot be compiled due to I forgot enable "wfe" config in the meson file,

so this issue can not happen before.

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Honestly, I don't understand why we have to remove perfectly valid 'const'

qualifier here.

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If this macro can't be used with pointers to const (still don't
understand why), then let's just not use this macro here.
Strictly speaking I don't see much benefit here from it.

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 {
-	uint32_t nuse, puse;
+	uint32_t puse;

 	/* make sure all previous loads and stores are completed */
 	rte_smp_mb();

@@ -122,11 +122,8 @@ bpf_eth_cbi_wait(const struct bpf_eth_cbi

*cbi)

 	/* in use, busy wait till current RX/TX iteration is finished */
 	if ((puse & BPF_ETH_CBI_INUSE) != 0) {
-		do {
-			rte_pause();
-			rte_compiler_barrier();
-			nuse = cbi->use;
-		} while (nuse == puse);
+		rte_wait_event(&cbi->use, UINT32_MAX, ==, puse,
+				__ATOMIC_RELAXED);

After another thought, if we do type conversion at macro invocation time:

bpf_eth_cbi_wait(const struct bpf_eth_cbi *cbi) {
  ...
  rte_wait_event((uint32_t *)&cbi->use, UINT32_MAX, ==, puse,
__ATOMIC_RELAXED);

would that help?

I try to with this and it will report compiler warning:
' cast discards ‘const’ qualifier'.

Something like:
(uint32_t *)(uintptr_t)&cbi->use
?

I think this is due to that in rte_wait_event macro, we use
typeof(*(addr)) value = 0;
 and value is defined as "const uint32_t",
but it should be able to be updated.
Furthermore, this reflects the limitations of the new macro, it cannot be applied
when 'addr' is type of 'const'. Finally, I think I should give up the change for "bpf".

Ah yes, I see.
One trick to avoid it:
typeof (*(addr) + 0) value;
...
But it would cause integer promotion for uint16_t.
So probably wouldn't suit you here.