netdev - Re: [PATCH net-next 01/14] qed: Add CONFIG_QED

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Date:	Tue, 10 May 2016 15:10:53 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	Yuval.Mintz@...gic.com
Cc:	netdev@...r.kernel.org, Ariel.Elior@...gic.com
Subject: Re: [PATCH net-next 01/14] qed: Add CONFIG_QED_SRIOV

From: Yuval Mintz <Yuval.Mintz@...gic.com>
Date: Tue, 10 May 2016 18:15:08 +0000

>> > I'm not entirely convinced this is true; If we'll not enforce the
>> > alignment of this 64-bit field, it's possible there will be
>> > differences between 32-bit and 64-bit machines versions of this struct.
>> > You have to recall that this is going to be copied via DMA between PF
>> > and VF, so they must have the exact same representation of the structure.
>> 
>> Then use properly sized types to fill in all the space in the structure, that's how
>> you guarantee layout, not aligned_u64.  Also, do not use the packed attribute.
>> 
>> struct foo {
>> 	u32 x;
>> 	u32 y;
>> 	u64 z;
>> };
>> 
>> 'z' will always be 64-bit aligned.
> 
> Perhaps my bit-numeric is a bit weak - why is it so?

foo is 64-bit aligned, therefore a properly aligned struct member will
be so as well.

We absolutely depend upon this for several data structures in the kernel.