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

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Message-ID: <CO2PR11MB0088ECDC1D7A65B703E2DDE797710@CO2PR11MB0088.namprd11.prod.outlook.com>
Date:	Tue, 10 May 2016 18:15:08 +0000
From:	Yuval Mintz <Yuval.Mintz@...gic.com>
To:	David Miller <davem@...emloft.net>
CC:	netdev <netdev@...r.kernel.org>,
	Ariel Elior <Ariel.Elior@...gic.com>
Subject: RE: [PATCH net-next 01/14] qed: Add CONFIG_QED_SRIOV

> > 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?
I.e., what prevents `z' from only being 32-bit aligned on a 32-bit machine?
Isn't it possible that (&x % 8)  == 4, (&y % 8) == 0 and (&z % 8) == 4 on such a platform?