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From: Serguei Tzukanov <tzukanov@narod.ru>
To: John Baldwin <jhb@FreeBSD.org>
Subject: Re: A question about S/390 port
Date:	Wed, 10 Jul 2002 20:33:25 +0400
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> td_retval[0] is the low word, and td_retval[1] is the high word, you
> just need to make sure the values from those two words get returned
> properly to userland.

1) syscall returns 32-bit value:
	r2 = rv[0];
	r3 = rv[1];
	r3 is irrelevant here (ABI: "32-bit values returned in r2")

2) syscall returns 64-bit value:
	MI code uses something like
	*(int64_t *)rv = xxx, so I have to do
	r2 = rv[0];
	r3 = rv[1];
	ABI says "long long shall be returned with the lower
	addressed half in r2 and the higher in r3"

3) syscall folded into __syscall returns 32-bit value (e.g. mmap):
	MI code does usual
	r[0] = xxx;
	svc (syscall) handler does
	r2 = rv[0];
	r3 = rv[1]; /* zeroed before */
	then mmap wrapper in userspace casts this 64-bit to 32-bit
	(loads r2 with r3 really) and always gets 0.

So to make it consistent I have to know size of returned value for every 
syscall and for case 3 do {r3 = rv[0]; r2 = rv[1]}.



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