From: "Robin Jarry" <rjarry@redhat.com>
To: "Tyler Retzlaff" <roretzla@linux.microsoft.com>
Cc: <dev@dpdk.org>, "Jerin Jacob" <jerinj@marvell.com>,
"Kiran Kumar K" <kirankumark@marvell.com>,
"Nithin Dabilpuram" <ndabilpuram@marvell.com>,
"Zhirun Yan" <yanzhirun_163@163.com>
Subject: Re: [PATCH v2] graph: expose node context as pointers
Date: Sat, 23 Mar 2024 00:41:51 +0100 [thread overview]
Message-ID: <D00ODGS68PT6.2CY67IB0P92D7@redhat.com> (raw)
In-Reply-To: <20240322165615.GA31848@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Hi Tyler,
Tyler Retzlaff, Mar 22, 2024 at 17:56:
> i can answer this!
>
> windows toolchains will only require __extension__ qualification on use
> of statement expressions, so msvc won't require any use of __extension__
> in this patch.
>
> as a general rule of thumb __extension__ is something you may choose to
> use for any gcc compiled code that is an extension to standard C and you
> intend to use the -pedantic flag (i.e. -std=c11 && -pedantic used together)
Got it, thanks!
> > /* Fast path area */
> > #define RTE_NODE_CTX_SZ 16
> > - alignas(RTE_CACHE_LINE_SIZE) uint8_t ctx[RTE_NODE_CTX_SZ]; /**< Node Context. */
> > + __extension__ alignas(RTE_CACHE_LINE_SIZE) union {
>
> __extension__ should not be on the anonymous union (since they are standard C11).
>
> anonymous union declaration is actually a type with no name and then a data
> field of that type so __rte_aligned is most likely what you want, since
> you're using RTE_CACHE_LINE_SIZE we can use __rte_cache_aligned.
>
> union __rte_cache_aligned {
> ... your union fields ...
> };
>
> and i think checkpatches still gives a warning unrelated to alignment
> for this but it can be safely ignored. it's the warning about alignment
> that we care about and should be fixed.
This passes the C++ header check but it breaks the static_assert I just
added. I believe the alignment is somehow transferred to all union
fields. And since ctx is an array, it makes the whole union .
So before my patch:
/* --- cacheline 3 boundary (192 bytes) --- */
uint8_t ctx[16] __attribute__((__aligned__(64))); /* 192 16 */
uint16_t size; /* 208 2 */
With the anonymous union aligned:
/* --- cacheline 3 boundary (192 bytes) --- */
union {
uint8_t ctx[16]; /* 192 16 */
struct {
void * ctx_ptr; /* 192 8 */
void * ctx_ptr2; /* 200 8 */
}; /* 192 16 */
} __attribute__((__aligned__(64))); /* 192 64 */
/* --- cacheline 4 boundary (256 bytes) --- */
uint16_t size; /* 256 2 */
However, if I remove the explicit align, I get what I expect:
/* --- cacheline 3 boundary (192 bytes) --- */
union {
uint8_t ctx[16]; /* 192 16 */
struct {
void * ctx_ptr; /* 192 8 */
void * ctx_ptr2; /* 200 8 */
}; /* 192 16 */
}; /* 192 16 */
uint16_t size; /* 208 2 */
Is it OK to drop the explicit alignment? This is beyond my C skills :)
> > + uint8_t ctx[RTE_NODE_CTX_SZ];
> > + /* Convenience aliases to store pointers without complex casting. */
> > + __extension__ struct {
>
> this is correct/recommended since anonymous structs aren't standard,
> with the __extension__ -pedantic won't emit a warning (our intention).
Ack.
> > +static_assert(offsetof(struct rte_node, size) - offsetof(struct rte_node, ctx) == RTE_NODE_CTX_SZ,
> > + "The node context anonymous union cannot be larger than RTE_NODE_CTX_SZ");
> > +
>
> you should include directly include <stddef.h> in this file for use of offsetof.
> you should include directly include <assert.h> in this file for use of the static_assert.
Will do for v3.
Thanks!
prev parent reply other threads:[~2024-03-22 23:41 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-22 16:31 Robin Jarry
2024-03-22 16:56 ` Tyler Retzlaff
2024-03-22 23:41 ` Robin Jarry [this message]
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