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From: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
To: Luc Pelletier <lucp.at.work@gmail.com>
Cc: "Richardson, Bruce" <bruce.richardson@intel.com>,
	"dev@dpdk.org" <dev@dpdk.org>,
	"Li, Xiaoyun" <xiaoyun.li@intel.com>,
	"stable@dpdk.org" <stable@dpdk.org>
Subject: RE: [PATCH v5] eal: fix unaligned loads/stores in rte_memcpy_generic
Date: Thu, 10 Feb 2022 14:04:34 +0000	[thread overview]
Message-ID: <DM6PR11MB4491E6A7BA3167268FD189B59A2F9@DM6PR11MB4491.namprd11.prod.outlook.com> (raw)
In-Reply-To: <CAFeRdtAgYLawMxk+mTfig--SCaTOPMX8tzNvV7Ed9qTfZJiUzA@mail.gmail.com>

Hi Luc,

> > > Calls to rte_memcpy_generic could result in unaligned loads/stores for
> > > 1 < n < 16. This is undefined behavior according to the C standard,
> > > and it gets flagged by the clang undefined behavior sanitizer.
> > >
> > > rte_memcpy_generic is called with unaligned src and dst addresses.
> > > When 1 < n < 16, the code would cast both src and dst to a qword,
> > > dword or word pointer, without verifying the alignment of src/dst. The
> > > code was changed to use a packed structure to perform the unaligned
> > > load/store operations. This results in unaligned load/store operations
> > > to be C standards-compliant.
> >
> > Still not sure we need to fix that:
> > This is x86 specific code-path, and as I remember on x86 there are no
> > penalties for unaligned access to 2/4/8 byte values.
> > Though I like introduction of rte_mov15_or_less() function -t helps
> > with code dedup.
> 
> Thanks for your input Konstantin. Much appreciated. Just to make sure
> I understand, can you please confirm that we do not want to fix the
> fact that unaligned access in C is undefined behaviour?

Yes, I don't think it is a real problem in that particular case.

> I understand
> that X86 allows unaligned accesses but since this isn't assembly code,
> we have to go by what the C standard allows.

> Also, do you have any opinion on the strict aliasing violation (as was
> pointed out by Georg)? I suggested adding __may_alias__ thinking it
> would likely fix the issue, but I'd really like to get confirmation
> from someone who has better knowledge of how the attribute works
> exactly.

Not sure I understand the problem you are referring to.
Are you saying that original rte_memcpy() code breaks strict aliasing? 
If so, could you point where exactly?


  reply	other threads:[~2022-02-10 14:04 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-15 19:41 [PATCH] " Luc Pelletier
2022-01-15 21:39 ` [PATCH v2] " Luc Pelletier
2022-01-15 22:13   ` Stephen Hemminger
2022-01-16 14:09     ` Luc Pelletier
2022-01-16 16:32       ` Stephen Hemminger
2022-01-16 14:13 ` [PATCH v3] " Luc Pelletier
2022-01-16 14:33   ` Luc Pelletier
2022-01-16 16:34     ` Stephen Hemminger
2022-01-16 17:59       ` Morten Brørup
2022-01-16 20:33 ` [PATCH v4] " Luc Pelletier
2022-01-17 15:37 ` [PATCH v5] " Luc Pelletier
2022-02-04 16:42   ` Luc Pelletier
2022-02-04 17:16   ` Ananyev, Konstantin
2022-02-08 16:53     ` Thomas Monjalon
2022-02-09 15:05     ` Luc Pelletier
2022-02-10 14:04       ` Ananyev, Konstantin [this message]
2022-02-10 16:56         ` Luc Pelletier
2022-02-11 15:51           ` Ananyev, Konstantin
2022-02-13 22:31             ` Luc Pelletier
2022-02-14 13:41               ` Ananyev, Konstantin
2022-02-25 15:51 ` [PATCH v6] eal: fix rte_memcpy strict aliasing/alignment bugs Luc Pelletier
2022-02-25 16:38 ` [PATCH v7] " Luc Pelletier
2022-03-10 14:55   ` Ananyev, Konstantin
2022-04-07 15:24     ` David Marchand
2022-04-07 15:32       ` David Marchand
2022-04-07 15:40         ` David Marchand
2022-05-13 19:15           ` Luc Pelletier
2022-05-19 16:41             ` David Marchand
2022-04-08 13:47       ` Luc Pelletier
2022-05-19 16:47       ` David Marchand

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