[PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number to the hypervisor
Russell King - ARM Linux
linux at arm.linux.org.uk
Thu Mar 1 10:12:58 UTC 2012
On Wed, Feb 29, 2012 at 02:44:24PM +0000, Ian Campbell wrote:
> > If you need a specific register, this means that you must set up that
> > register explicitly inside the asm if you want a guarantee that the
> > code will work:
> > asm volatile (
> > "movw r12, %[hvc_num]\n\t"
> Is gcc (or gas?) smart enough to optimise this away if it turns out that
> %[hvc_num] == r12?
No, and it won't do, because %[hvc_num] is specified in these operands:
> > ...
> > "hvc #0"
> > :: [hvc_num] "i" (NUMBER) : "r12"
to be an integer, not a register.
> How are system calls implemented on the userspace side? I confess I
> don't know what the ARM syscall ABI looks like -- is it all registers or
> is some of it on the stack? It sounds like the solution ought to be
> pretty similar though.
All registers. We have a few which take a pointer to an in memory array,
but those are for some old multiplexed syscalls.
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