Physical memory start contraints in the Linux kernel (Was: Re: Xen osstest on Calxeda midway progress (Was: Re: [Xen-devel] [xen-unstable test] 21486: tolerable FAIL - PUSHED))

[Stefano Stabellini]

On Wed, 13 Nov 2013, Arnd Bergmann wrote:
> On Wednesday 13 November 2013, Stefano Stabellini wrote:
> >  
> > -#define xchg_xen_ulong(ptr, val) atomic64_xchg(container_of((ptr),	\
> > +#ifdef CONFIG_GENERIC_ATOMIC64
> > +/* if CONFIG_GENERIC_ATOMIC64 is defined we cannot use the generic
> > + * atomic64_xchg function because it is implemented using spin locks.
> > + * Here we need proper atomic instructions to read and write memory
> > + * shared with the hypervisor.
> > + */
> > +static inline u64 xen_atomic64_xchg(atomic64_t *ptr, u64 new)
> > +{
> > +	u64 result;
> > +	unsigned long tmp;
> > +
> > +	smp_mb();
> > +
> > +	__asm__ __volatile__("@ xen_atomic64_xchg\n"
> > +"1:	ldrexd	%0, %H0, [%3]\n"
> > +"	strexd	%1, %4, %H4, [%3]\n"
> > +"	teq	%1, #0\n"
> > +"	bne	1b"
> > +	: "=&r" (result), "=&r" (tmp), "+Qo" (ptr->counter)
> > +	: "r" (&ptr->counter), "r" (new)
> > +	: "cc");
> > +
> > +	smp_mb();
> > +
> > +	return result;
> > +}
> > +#else
> > +#define xen_atomic64_xchg atomic64_xchg
> > +#endif
> 
> I would prefer not duplicating this code. Maybe we can instead change
> the ARM code to always provide this function under the name of
> armv7_atomic64_xchg() and conditionally defining atomic64_xchg
> to use it. Let's see if Russell has an opinion on this, or perhaps
> a better solution.
> 
> > diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> > index 62ccf54..d668c3c 100644
> > --- a/drivers/xen/grant-table.c
> > +++ b/drivers/xen/grant-table.c
> > @@ -33,6 +33,14 @@
> >  
> >  #define pr_fmt(fmt) "xen:" KBUILD_MODNAME ": " fmt
> >  
> > +/* This is required because cmpxchg only support 32-bits operands on
> > + * ARMv6, so if CONFIG_CPU_V6 is defined the cmpxchg implemention will
> > + * be limited to 32-bits operands.
> > + * However we know for sure that if Linux is running on Xen, the
> > + * platform is >= ARMv7, so here we can safely undef CONFIG_CPU_V6.
> > + */
> > +#undef CONFIG_CPU_V6
> > +
> >  #include <linux/module.h>
> >  #include <linux/sched.h>
> >  #include <linux/mm.h>
> > 
> 
> Same here: I'd prefer making this more explicit by changing the cmpxchg
> implementation: we could split out the 16-bit cmpxchg case into a separate
> function with "armv7" in the name and only call that from the main
> cmpxchg() implementations when building an armv7-only kernel, but still
> let you call it from Xen code that is known to run only on armv7.
 
what do you think of:

http://marc.info/?l=linux-arm-kernel&m=138444245625671&w=2

?