linux-kernel - Re: [PATCH 1/6] GenWQE PCI support, health monitoring and recovery

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Message-Id: <201312052131.42316.arnd@arndb.de>
Date:	Thu, 5 Dec 2013 21:31:42 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	haver@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
	cody@...ux.vnet.ibm.com, schwidefsky@...ibm.com,
	utz.bacher@...ibm.com, mmarek@...e.cz, rmallon@...il.com,
	jsvogt@...ibm.com, MIJUNG@...ibm.com, cascardo@...ux.vnet.ibm.com,
	michael@...ra.de
Subject: Re: [PATCH 1/6] GenWQE PCI support, health monitoring and recovery

On Thursday 05 December 2013, Frank Haverkamp wrote:
> > > Was wrong, as already pointed out before. It is now:
> > > 
> > > struct genwqe_mem {
> > >     __u64 addr;
> > >     __u64 size;
> > >     int direction;
> > > };
> > > 
> > > I hope the int is ok here.
> > 
> > No, it's not. The problem is that sizeof(struct genwqe_mem) is now 24 on
> > most architectures (including x86-64) and 20 on x86-32.
> 
> Interesting. So int is like long architecture specific. I changed it to
> be __u64 too, to avoid any problem.

The solution is ok, but the problem is different from what you thought:

On all architectures that Linux runs on, 'int' is 32 bit. The problem is
again the alignment of __u64. On normal architectures, it is naturally
aligned, and gcc adds 4 byte padding so that 'sizeof (struct genwqe_mem)'
is  multiple of the required alignment. On x86-32, the required alignment
for the __u64 members is only 4 bytes, so no padding is added.

	Arnd
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