linux-kernel - Re: [Xen-devel] [PATCH 01/24] arm: initial Xen support

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Message-ID: <alpine.DEB.2.02.1208061146480.4645@kaball.uk.xensource.com>
Date:	Mon, 6 Aug 2012 11:55:38 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	"arnd@...db.de" <arnd@...db.de>,
	"catalin.marinas@....com" <catalin.marinas@....com>,
	"Tim (Xen.org)" <tim@....org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH 01/24] arm: initial Xen support

On Wed, 1 Aug 2012, Konrad Rzeszutek Wilk wrote:
> > > > +struct pvclock_wall_clock {
> > > > +	u32   version;
> > > > +	u32   sec;
> > > > +	u32   nsec;
> > > > +} __attribute__((__packed__));
> > > 
> > > That is weird. It is 4+4+4 = 12 bytes? Don't you want it to be 16 bytes?
> > 
> > I agree that 16 bytes would be a better choice, but it needs to match
> > the struct in Xen that is defined as follow:
> > 
> >     uint32_t wc_version;      /* Version counter: see vcpu_time_info_t. */
> >     uint32_t wc_sec;          /* Secs  00:00:00 UTC, Jan 1, 1970.  */
> >     uint32_t wc_nsec;         /* Nsecs 00:00:00 UTC, Jan 1, 1970.  */
> 
> Would it make sense to add some paddigin then at least? In both
> cases? Or is it too late for this?

I can see why adding some padding would be useful if the structs were
not packed and we wanted to enforce 32/64 bit compatibility on x86.
However on ARM the field alignments on 32 and 64 bits are the same for
integer values so the padding wouldn't make a difference.
In any case both structs are packed, so the alignment is forced to be the
same by the compiler.
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