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

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Message-ID: <1362475797.8941.67.camel@hastur.hellion.org.uk>
Date:	Tue, 5 Mar 2013 09:29:57 +0000
From:	Ian Campbell <ian.campbell@...rix.com>
To:	Will Deacon <will.deacon@....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

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.

Ian.

diff --git a/arch/arm/include/asm/xen/events.h b/arch/arm/include/asm/xen/events.h
index 0e1f59e..e86a1b3 100644
--- a/arch/arm/include/asm/xen/events.h
+++ b/arch/arm/include/asm/xen/events.h
@@ -2,6 +2,7 @@
 #define _ASM_ARM_XEN_EVENTS_H
 
 #include <asm/ptrace.h>
+#include <asm/atomic.h>
 
 enum ipi_vector {
 	XEN_PLACEHOLDER_VECTOR,
@@ -15,27 +16,6 @@ static inline int xen_irqs_disabled(struct pt_regs *regs)
 	return raw_irqs_disabled_flags(regs->ARM_cpsr);
 }
 
-/*
- * We cannot use xchg because it does not support 8-byte
- * values. However it is safe to use {ldr,dtd}exd directly because all
- * platforms which Xen can run on support those instructions.
- */
-static inline xen_ulong_t xchg_xen_ulong(xen_ulong_t *ptr, xen_ulong_t val)
-{
-	xen_ulong_t oldval;
-	unsigned int tmp;
-
-	smp_wmb();
-	asm volatile("@ xchg_xen_ulong\n"
-		"1:     ldrexd  %0, %H0, [%3]\n"
-		"       strexd  %1, %2, %H2, [%3]\n"
-		"       teq     %1, #0\n"
-		"       bne     1b"
-		: "=&r" (oldval), "=&r" (tmp)
-		: "r" (val), "r" (ptr)
-		: "memory", "cc");
-	smp_wmb();
-	return oldval;
-}
+#define xchg_xen_ulong(ptr, val) atomic64_xchg((atomic64_t *)(ptr), (val))
 
 #endif /* _ASM_ARM_XEN_EVENTS_H */


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