On 12.12.2012, at 21:12, Christian Borntraeger wrote:
> 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.
How so? The asm/ccwdev.h header simply says
struct subchannel_id;
with the actual implementation of that struct hidden in asm/schid.h. So either patch 3/4 exports the struct needlessly and should rather do an #include <asm/schid.h> or the compiler can do magic :).
Alex
>
> 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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