Re: [dpdk-dev] [PATCH] eal: fix 64bit address alignment in 32-bit builds

[Thomas Monjalon <thomas@monjalon.net>]

28/04/2017 11:03, Bruce Richardson:
> On Fri, Apr 28, 2017 at 10:56:56AM +0200, Thomas Monjalon wrote:
> > 28/04/2017 10:15, Bruce Richardson:
> > > On i686 builds, the uint64_t type is 64-bits in size but is aligned to
> > > 32-bits only. This causes mbuf fields for rearm_data to not be 16-byte
> > > aligned on 32-bit builds, which causes errors with some vector PMDs which
> > > expect the rearm data to be aligned as on 64-bit.
> > > 
> > > Given that we cannot use the extra space in the data structures anyway, as
> > > it's already used on 64-bit builds, we can just force alignment of physical
> > > address structure members to 8-bytes in all cases. This has no effect on
> > > 64-bit systems, but fixes the updated PMDs on 32-bit.
> > 
> > I agree to align on 64-bit in mbuf.
> > 
> > > Fixes: f4356d7ca168 ("net/i40e: eliminate mbuf write on rearm")
> > > Fixes: f160666a1073 ("net/ixgbe: eliminate mbuf write on rearm")
> > [...]
> > > --- a/lib/librte_eal/common/include/rte_memory.h
> > > +++ b/lib/librte_eal/common/include/rte_memory.h
> > > -typedef uint64_t phys_addr_t; /**< Physical address definition. */
> > > +/** Physical address definition. */
> > > +typedef uint64_t phys_addr_t __rte_aligned(sizeof(uint64_t));
> > 
> > Why setting this constraint for everyone?
> >
> Well, it only has an effect on 32-bit builds, and unless there is a
> problem, I don't see why not always align them to the extra 8 bytes. If
> this does cause an issue, I'm happy enough to use #ifdefs, but in the
> absense of a confirmed problem, I'd rather keep the code clean.

Is it expected for everyone to have every physical addresses aligned on 64?
I think it can be weird for some applications.
Why do you think it is cleaner than adding the alignment to the mbuf fields?

PS: It is yet another macro which is not rte_ prefixed.