[Xen-devel] [PATCH 5/9] ARM: Xen: fix initial build problems:

On Tuesday 09 October 2012, Ian Campbell wrote:

On Tue, 2012-10-09 at 16:39 +0100, Arnd Bergmann wrote:

quoted

On Tuesday 09 October 2012, Ian Campbell wrote:

quoted

quoted

* The tmem hypercall is not available on ARM

* ARMv6 does not support cmpxchg on 16-bit words that are used in the

missing the end of this sentence?

Right, I meant to say 

* ARMv6 does not support cmpxchg on 16-bit words that are used in the
  Xen grant table code, so we must ensure that Xen support is only built
  on ARMv7-only kernels not combined ARMv6/v7 kernels.

This should be fixed differently in the future.

Is this is a build time failure because gcc/gas/etc refuses to generate
these instructions if it is configured for v6?

I ask because if it is only a runtime issue then we can reason that if
we are running Xen specific grant table code, then we must be running on
Xen and therefore must necessarily be running on a v7 (because Xen only
support v7+virt extensions) even if the kernel happens to be capable of
running on v6 too.

The underlying reason of course is that ARMv6 doesn't have those
instructions. The symptom you see is a link error because gcc emits
a reference to the (intentionally missing) __bad_cmpxchg() function
from

static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
                                      unsigned long new, int size)
{
        unsigned long oldval, res;

        switch (size) {
#ifndef CONFIG_CPU_V6   /* min ARCH >= ARMv6K */
        case 1:
		...
                break;
        case 2:
		...
                break;
#endif
        case 4:
		...
                break;
        default:
                __bad_cmpxchg(ptr, size);
                oldval = 0;
        }
        return oldval;
}

The possible solutions I can see for this are:

* change the grant table format to use 32 bits for the flags on ARM
* change the code to always cmpxchg the entire 32 bit word including the flags.
* implement your own cmpxchg wrapper that may be implemented using a spinlock
  rather than cmpxchg if ARMv6 is enabled.

	Arnd