On 12/12/12 19:31, Alexander Graf wrote:
+static inline long do_kvm_notify(struct subchannel_id schid,
>> + unsigned long queue_index)
>> +{
>> + register unsigned long __nr asm("1") = KVM_S390_VIRTIO_CCW_NOTIFY;
>> + register struct subchannel_id __schid asm("2") = schid;
>
> How does the compiler know that the struct fits within a register?
>
Well it knows that sizeof struct subchannel_id is 4.
then we have
"ELF Application Binary Interface Supplement Linux on zSeries"
[...]
Parameter passing
[...]
- Structures with a size of 1, 2, 4, or 8 bytes are passed as integral values. <=============
- All other structures are passed by reference. If needed, the called function makes
a copy of the value.
[...]
while not being a perfect match it gives a good indication that this will work.
Furthermore, we know it is working at least since
commit 94c12cc7d196bab34aaa98d38521549fa1e5ef76
Author: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Date: Thu Sep 28 16:56:43 2006 +0200
[S390] Inline assembly cleanup.
in
drivers/s390/cio/ioasm.h
[...]
94c12cc7 (Martin Schwidefsky 2006-09-28 16:56:43 +0200 30) register struct subchannel_id reg1 asm ("1") = schid;
94c12cc7 (Martin Schwidefsky 2006-09-28 16:56:43 +0200 31) int ccode = -EIO;
fb6958a5 (Cornelia Huck 2006-01-06 00:19:25 -0800 32)
94c12cc7 (Martin Schwidefsky 2006-09-28 16:56:43 +0200 33) asm volatile(
[...]
So this complies with the coding style of the s390 maintainer :-)
Christian
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