alignment faults in 3.6
Russell King - ARM Linux
linux at arm.linux.org.uk
Fri Oct 5 04:24:39 EDT 2012
On Fri, Oct 05, 2012 at 09:20:56AM +0100, Mans Rullgard wrote:
> On 5 October 2012 08:12, Russell King - ARM Linux
> <linux at arm.linux.org.uk> wrote:
> > On Fri, Oct 05, 2012 at 03:25:16AM +0100, Mans Rullgard wrote:
> >> On 5 October 2012 02:56, Rob Herring <robherring2 at gmail.com> wrote:
> >> > This struct is the IP header, so a struct ptr is just set to the
> >> > beginning of the received data. Since ethernet headers are 14 bytes,
> >> > often the IP header is not aligned unless the NIC can place the frame at
> >> > a 2 byte offset (which is something I need to investigate). So this
> >> > function cannot make any assumptions about the alignment. Does the ABI
> >> > define structs have some minimum alignment? Does the struct need to be
> >> > declared as packed or something?
> >> The ABI defines the alignment of structs as the maximum alignment of its
> >> members. Since this struct contains 32-bit members, the alignment for the
> >> whole struct becomes 32 bits as well. Declaring it as packed tells gcc it
> >> might be unaligned (in addition to removing any holes within).
> > This has come up before in the past.
> > The Linux network folk will _not_ allow - in any shape or form - for
> > this struct to be marked packed (it's the struct which needs to be
> > marked packed) because by doing so, it causes GCC to issue byte loads/
> > stores on architectures where there isn't a problem, and that decreases
> > the performance of the Linux IP stack unnecessarily.
> Which architectures? I have never seen anything like that.
Does it matter? I'm just relaying the argument against adding __packed
which was used before we were forced (by the networking folk) to implement
the alignment fault handler.
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