s390/qdio: fix do_sqbs() inline assembly constraint

Use "a" constraint instead of "d" constraint to pass the state parameter to
the do_sqbs() inline assembly. This prevents that general purpose register
zero is used for the state parameter.

If the compiler would select general purpose register zero this would be
problematic for the used instruction in rsy format: the register used for
the state parameter is a base register. If the base register is general
purpose register zero the contents of the register are unexpectedly ignored
when the instruction is executed.

This only applies to z/VM guests using QIOASSIST with dedicated (pass through)
QDIO-based devices such as FCP [zfcp driver] as well as real OSA or
HiperSockets [qeth driver].

A possible symptom for this case using zfcp is the following repeating kernel
message pattern:

zfcp <devbusid>: A QDIO problem occurred
zfcp <devbusid>: A QDIO problem occurred
zfcp <devbusid>: qdio: ZFCP on SC <sc> using AI:1 QEBSM:1 PRI:1 TDD:1 SIGA: W
zfcp <devbusid>: A QDIO problem occurred
zfcp <devbusid>: A QDIO problem occurred

Each of the qdio problem message can be accompanied by the following entries
for the affected subchannel <sc> in
/sys/kernel/debug/s390dbf/qdio_error/hex_ascii for zfcp or qeth:

<sc> ccq: 69....
<sc> SQBS ERROR.

Reviewed-by: Benjamin Block <[email protected]>
Cc: Steffen Maier <[email protected]>
Fixes: 8129ee1 ("[PATCH] s390: qdio V=V pass-through")
Cc: <[email protected]>
Signed-off-by: Heiko Carstens <[email protected]>
Signed-off-by: Alexander Gordeev <[email protected]>

Author: hcahca

drivers/s390/cio/qdio.h

@@ -95,7 +95,7 @@ static inline int do_sqbs(u64 token, unsigned char state, int queue,
 		"	lgr	1,%[token]\n"
 		"	.insn	rsy,0xeb000000008a,%[qs],%[ccq],0(%[state])"
 		: [ccq] "+&d" (_ccq), [qs] "+&d" (_queuestart)
-		: [state] "d" ((unsigned long)state), [token] "d" (token)
+		: [state] "a" ((unsigned long)state), [token] "d" (token)
 		: "memory", "cc", "1");
 	*count = _ccq & 0xff;
 	*start = _queuestart & 0xff;