[PATCH v1] s390x/tcg: Fix RISBHG

[David Hildenbrand <david@redhat.com>]

RISBHG is broken and currently hinders clang builds of upstream kernels
from booting: the kernel crashes early, while decompressing the image.

  [...]
   Kernel fault: interruption code 0005 ilc:2
   Kernel random base: 0000000000000000
   PSW : 0000200180000000 0000000000017a1e
         R:0 T:0 IO:0 EX:0 Key:0 M:0 W:0 P:0 AS:0 CC:2 PM:0 RI:0 EA:3
   GPRS: 0000000000000001 0000000c00000000 00000003fffffff4 00000000fffffff0
         0000000000000000 00000000fffffff4 000000000000000c 00000000fffffff0
         00000000fffffffc 0000000000000000 00000000fffffff8 00000000008e25a8
         0000000000000009 0000000000000002 0000000000000008 000000000000bce0

One example of a buggy instruction is:

    17dde:       ec 1e 00 9f 20 5d       risbhg  %r1,%r14,0,159,32

With %r14 = 0x9 and %r1 = 0x7 should result in %r1 = 0x900000007, however,
results in %r1 = 0.

Let's interpret values of i3/i4 as documented in the PoP and make
computation of "mask" only based on i3 and i4 and use "pmask" only at the
very end to make sure wrapping is only applied to the high/low doubleword.

With this patch, I can successfully boot a v5.10 kernel built with
clang, and gcc builds keep on working.

Fixes: 2d6a869833d9 ("target-s390: Implement RISBG")
Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---

This BUG was a nightmare to debug and the code a nightmare to understand.

To make clang/gcc builds boot, the following fix is required as well on
top of current master: "[PATCH] target/s390x: Fix ALGSI"
https://lkml.kernel.org/r/20210107202135.52379-1-david@redhat.com

---
 target/s390x/translate.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/target/s390x/translate.c b/target/s390x/translate.c
index 3d5c0d6106..39e33eeb67 100644
--- a/target/s390x/translate.c
+++ b/target/s390x/translate.c
@@ -3815,22 +3815,23 @@ static DisasJumpType op_risbg(DisasContext *s, DisasOps *o)
         pmask = 0xffffffff00000000ull;
         break;
     case 0x51: /* risblg */
-        i3 &= 31;
-        i4 &= 31;
+        i3 = (i3 & 31) + 32;
+        i4 = (i4 & 31) + 32;
         pmask = 0x00000000ffffffffull;
         break;
     default:
         g_assert_not_reached();
     }
 
-    /* MASK is the set of bits to be inserted from R2.
-       Take care for I3/I4 wraparound.  */
-    mask = pmask >> i3;
+    /* MASK is the set of bits to be inserted from R2. */
     if (i3 <= i4) {
-        mask ^= pmask >> i4 >> 1;
+        /* [0...i3---i4...63] */
+        mask = (-1ull >> i3) & (-1ull << (63 - i4));
     } else {
-        mask |= ~(pmask >> i4 >> 1);
+        /* [0---i4...i3---63] */
+        mask = (-1ull >> i3) | (-1ull << (63 - i4));
     }
+    /* For RISBLG/RISBHG, the wrapping is limited to the high/low doubleword. */
     mask &= pmask;
 
     /* IMASK is the set of bits to be kept from R1.  In the case of the high/low
@@ -3843,9 +3844,6 @@ static DisasJumpType op_risbg(DisasContext *s, DisasOps *o)
     len = i4 - i3 + 1;
     pos = 63 - i4;
     rot = i5 & 63;
-    if (s->fields.op2 == 0x5d) {
-        pos += 32;
-    }
 
     /* In some cases we can implement this with extract.  */
     if (imask == 0 && pos == 0 && len > 0 && len <= rot) {
-- 
2.29.2