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From: David Marchand <david.marchand@redhat.com>
Date: Tue, 5 Mar 2024 15:30:49 +0100
Message-ID: <CAJFAV8wAcDT3_jeVZSiDi6eMWRWVvgvVr+-63Ker_YMCtcGCEA@mail.gmail.com>
Subject: Re: [PATCH v7 08/39] mbuf: use C11 alignas
To: Tyler Retzlaff <roretzla@linux.microsoft.com>
Cc: dev@dpdk.org, Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>,
Bruce Richardson <bruce.richardson@intel.com>,
Chengwen Feng <fengchengwen@huawei.com>,
Cristian Dumitrescu <cristian.dumitrescu@intel.com>,
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On Mon, Mar 4, 2024 at 6:54=E2=80=AFPM Tyler Retzlaff
<roretzla@linux.microsoft.com> wrote:
>
> The current location used for __rte_aligned(a) for alignment of types
> and variables is not compatible with MSVC. There is only a single
> location accepted by both toolchains.
>
> For variables standard C11 offers alignas(a) supported by conformant
> compilers i.e. both MSVC and GCC.
>
> For types the standard offers no alignment facility that compatibly
> interoperates with C and C++ but may be achieved by relocating the
> placement of __rte_aligned(a) to the aforementioned location accepted
> by all currently supported toolchains.
>
> To allow alignment for both compilers do the following:
>
> * Move __rte_aligned from the end of {struct,union} definitions to
> be between {struct,union} and tag.
>
> The placement between {struct,union} and the tag allows the desired
> alignment to be imparted on the type regardless of the toolchain being
> used for all of GCC, LLVM, MSVC compilers building both C and C++.
>
> * Replace use of __rte_aligned(a) on variables/fields with alignas(a).
>
> Signed-off-by: Tyler Retzlaff <roretzla@linux.microsoft.com>
> Acked-by: Morten Br=C3=B8rup <mb@smartsharesystems.com>
> Acked-by: Konstantin Ananyev <konstantin.ananyev@huawei.com>
> ---
> lib/mbuf/rte_mbuf_core.h | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/lib/mbuf/rte_mbuf_core.h b/lib/mbuf/rte_mbuf_core.h
> index 5688683..917a811 100644
> --- a/lib/mbuf/rte_mbuf_core.h
> +++ b/lib/mbuf/rte_mbuf_core.h
> @@ -463,7 +463,7 @@ enum {
> /**
> * The generic rte_mbuf, containing a packet mbuf.
> */
> -struct rte_mbuf {
> +struct __rte_cache_aligned rte_mbuf {
> RTE_MARKER cacheline0;
>
> void *buf_addr; /**< Virtual address of segment buffer.=
*/
> @@ -476,7 +476,7 @@ struct rte_mbuf {
> * same mbuf cacheline0 layout for 32-bit and 64-bit. This makes
> * working on vector drivers easier.
> */
> - rte_iova_t buf_iova __rte_aligned(sizeof(rte_iova_t));
> + alignas(sizeof(rte_iova_t)) rte_iova_t buf_iova;
> #else
> /**
> * Next segment of scattered packet.
> @@ -662,7 +662,7 @@ struct rte_mbuf {
> uint16_t timesync;
>
> uint32_t dynfield1[9]; /**< Reserved for dynamic fields. */
> -} __rte_cache_aligned;
> +};
I probably missed the discussion, but why is cacheline1 not handled in
this patch?
I was expecting a:
- RTE_MARKER cacheline1 __rte_cache_min_aligned;
+ alignas(RTE_CACHE_LINE_MIN_SIZE) RTE_MARKER cacheline1;
--=20
David Marchand