alignment faults in 3.6
mans.rullgard at linaro.org
Fri Oct 5 04:33:04 EDT 2012
On 5 October 2012 09:24, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> 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?
It matters if we want to fix it.
Mans Rullgard / mru
More information about the linux-arm-kernel