RE: [PATCH v5 01/16] eal: provide pack start macro for MSVC

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> -----Original Message-----
> From: Morten Brørup <mb@smartsharesystems.com>
> Sent: Tuesday, November 19, 2024 8:32 AM
> To: Andre Muezerie <andremue@linux.microsoft.com>; roretzla@linux.microsoft.com; techboard@dpdk.org
> Cc: Yuying.Zhang@intel.com; aman.deep.singh@intel.com; anatoly.burakov@intel.com; bruce.richardson@intel.com;
> byron.marohn@intel.com; conor.walsh@intel.com; cristian.dumitrescu@intel.com; david.hunt@intel.com; dev@dpdk.org;
> dsosnowski@nvidia.com; gakhil@marvell.com; jerinj@marvell.com; jingjing.wu@intel.com; kirill.rybalchenko@intel.com;
> konstantin.v.ananyev@yandex.ru; matan@nvidia.com; orika@nvidia.com; radu.nicolau@intel.com; ruifeng.wang@arm.com;
> sameh.gobriel@intel.com; sivaprasad.tummala@amd.com; skori@marvell.com; stephen@networkplumber.org;
> suanmingm@nvidia.com; vattunuru@marvell.com; viacheslavo@nvidia.com; vladimir.medvedkin@intel.com;
> yipeng1.wang@intel.com
> Subject: RE: [PATCH v5 01/16] eal: provide pack start macro for MSVC
> 
> > From: Andre Muezerie [mailto:andremue@linux.microsoft.com]
> > Sent: Tuesday, 19 November 2024 05.35
> >
> > From: Tyler Retzlaff <roretzla@linux.microsoft.com>
> >
> > MSVC struct packing is not compatible with GCC. Provide a macro that
> > can be used to push existing pack value and sets packing to 1-byte.
> > The existing __rte_packed macro is then used to restore the pack value
> > prior to the push.
> >
> > Instead of providing macros exclusively for MSVC and for GCC the
> > existing macro is deliberately utilized to trigger a warning if no
> > existing packing has been pushed allowing easy identification of
> > locations where the __rte_msvc_pack is missing.
> >
> > Signed-off-by: Tyler Retzlaff <roretzla@linux.microsoft.com>
> > ---
> >  lib/eal/include/rte_common.h | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/lib/eal/include/rte_common.h
> > b/lib/eal/include/rte_common.h
> > index 4d299f2b36..409890863e 100644
> > --- a/lib/eal/include/rte_common.h
> > +++ b/lib/eal/include/rte_common.h
> > @@ -103,8 +103,10 @@ typedef uint16_t unaligned_uint16_t;
> >   * Force a structure to be packed
> >   */
> >  #ifdef RTE_TOOLCHAIN_MSVC
> > -#define __rte_packed
> > +#define __rte_msvc_pack __pragma(pack(push, 1))
> > +#define __rte_packed __pragma(pack(pop))
> >  #else
> > +#define __rte_msvc_pack
> >  #define __rte_packed __attribute__((__packed__))
> >  #endif
> >
> > --
> > 2.47.0.vfs.0.3
> 
> Before proceeding with this, can we please discuss the alternative, proposed here:
> https://inbox.dpdk.org/dev/CAJFAV8yStgiBbe+Nkt9mC30r0+ZP64_kGuRHOzqd90RD2HXZyw@mail.gmail.com/
> 
> The definition of the packing macro in OVS, for reference:
> https://github.com/openvswitch/ovs/blob/main/include/openvswitch/compiler.h#L209

Yes, that one looks much nicer to me too.
Could I also ask to avoid creeping _msvc_ into DPDK namespace.

> 
> The current solution requires __rte_packed to be placed at the end of a structure, although __attribute__((packed)) is normally
> allowed at the beginning (between the "struct" tag and the name of the structure), which introduces a high risk of contributors placing
> it "incorrectly", thus causing errors.
> 
> I have a strong preference for an __RTE_PACKED(decl) variant.
> 
> Here's a third alternative:
> #ifdef RTE_TOOLCHAIN_MSVC
> #define __rte_msvc_pack_begin __pragma(pack(push, 1))
> #define __rte_msvc_pack_end   __pragma(pack(pop))
> #else
> #define __rte_msvc_pack_begin
> #define __rte_msvc_pack_end
> #endif
> 
> The third alternative is also problematic, e.g. if a contributor forgets the _end after the structure declaration, or adds another
> structure declaration before the _end.
> 
> -Morten