linux-kernel - Re: [PATCH LINUX v5] xen: event channel arrays are xen_ulong

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Message-ID: <20130307031707.GA25137@mudshark.cambridge.arm.com>
Date:	Thu, 7 Mar 2013 03:17:07 +0000
From:	Will Deacon <will.deacon@....com>
To:	Ian Campbell <ian.campbell@...rix.com>
Cc:	Rob Herring <robherring2@...il.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"Keir (Xen.org)" <keir@....org>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"Tim (Xen.org)" <tim@....org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jan Beulich <JBeulich@...e.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Nicolas Pitre <nico@...xnic.net>
Subject: Re: [PATCH LINUX v5] xen: event channel arrays are xen_ulong_t and
 not unsigned long

Hi Ian,

On Tue, Mar 05, 2013 at 09:29:57AM +0000, Ian Campbell wrote:
> On Tue, 2013-03-05 at 08:08 +0000, Will Deacon wrote:
> > Cheers Rob, that was enough to reproduce for me. The problem is likely that
> > CONFIG_AEABI=n, so the ABI doesn't actually mandate even base registers for
> > 64-bit values in registers.
> 
> Me too.
> 
> > Ian -- this would be fixed if you used our atomic64 routines instead of
> > inventing your own :)
> 
> I looked and couldn't see an existing 64 bit xchg, was I looking in the
> wrong place? Ah, wait, I see atomic64_xchg now. But that needs an
> atomic64_t while I have a xen_ulong_t (which == 64 bits on ARM). This is
> a kernel<->hypervisor ABI so I can't just change it to an atomic64_t. I
> suppose I could cast (see below, untested) but that seems rather icky.

You can play some container_of tricks, like we do in cmpxchg64 to get this
right. Alternatively, we could look at an xchg8 implementation which some
other architectures have (although they seem to be 64-bit machines).

Will
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