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

[Andre Muezerie <andremue@linux.microsoft.com>]

On Tue, Nov 19, 2024 at 11:00:56AM +0000, Konstantin Ananyev wrote:
> 
> 
> > -----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.
> 

These are all good comments. I'll look more into it.

> > 
> > 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