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

[Olivier Matz <olivier.matz@6wind.com>]

Hi,

On Fri, 28 Apr 2017 10:32:03 +0100, Bruce Richardson <bruce.richardson@intel.com> wrote:
> On Fri, Apr 28, 2017 at 11:21:27AM +0200, Thomas Monjalon wrote:
> > 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?
> >   
> I'm ok to redo the patch to only make the change to the mbuf value.
> However, when researching this, I discovered that gcc apparently already
> aligns all non-structure-member uint64_t values on an 8-byte boundary on
> 32-bit x86 anyway*. [Don't know if this also applies e.g. to 32-bit arm,
> but I wouldn't be surprised if it did.] That means the scope of this
> only applies to structures with phys_addr values, so it's not a huge
> scope.
> *Ref: https://gcc.gnu.org/ml/gcc/2009-06/msg00333.html
> 
> > PS: It is yet another macro which is not rte_ prefixed.
> >   
> Yes. Not going to fix that in this patch though!
> 
> So, do you want a V2 to limit the alignment change to the phys_addr in
> the mbuf, rather than generally to physical addresses? I prefer the way
> I have it here, but I'm ok to change.

Since the need comes from vector pmd, I think it's better to limit
the alignment in the mbuf.

Also, it would be good to progressively add some compile-time BUG_ON() in
vector PMDs that have some hidden field alignment/ordering constraints.

Olivier