Merge remote-tracking branch 'remotes/riscv/tags/riscv-qemu-2.12-critical-fixes' into staging

RISC-V: Critical fixes for QEMU 2.12

This series includes changes that are considered release critical,
such as floating point register file corruption under SMP Linux
due to incorrect handling of mstatus.FS.

This workaround will be replaced with a more comprehensive fix
for mstatus.FS handling in QEMU 2.13.

# gpg: Signature made Thu 29 Mar 2018 18:22:42 BST
# gpg:                using DSA key 6BF1D7B357EF3E4F
# gpg: Good signature from "Michael Clark <michaeljclark@mac.com>"
# gpg:                 aka "Michael Clark <mjc@sifive.com>"
# gpg:                 aka "Michael Clark <michael@metaparadigm.com>"
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* remotes/riscv/tags/riscv-qemu-2.12-critical-fixes:
  RISC-V: Workaround for critical mstatus.FS bug
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target/riscv/op_helper.c

@@ -144,8 +144,21 @@ void csr_write_helper(CPURISCVState *env, target_ulong val_to_write,
         }
 
         mstatus = (mstatus & ~mask) | (val_to_write & mask);
-        int dirty = (mstatus & MSTATUS_FS) == MSTATUS_FS;
-        dirty |= (mstatus & MSTATUS_XS) == MSTATUS_XS;
+
+        /* Note: this is a workaround for an issue where mstatus.FS
+           does not report dirty after floating point operations
+           that modify floating point state. This workaround is
+           technically compliant with the RISC-V Privileged
+           specification as it is legal to return only off, or dirty.
+           at the expense of extra floating point save/restore. */
+
+        /* FP is always dirty or off */
+        if (mstatus & MSTATUS_FS) {
+            mstatus |= MSTATUS_FS;
+        }
+
+        int dirty = ((mstatus & MSTATUS_FS) == MSTATUS_FS) |
+                    ((mstatus & MSTATUS_XS) == MSTATUS_XS);
         mstatus = set_field(mstatus, MSTATUS_SD, dirty);
         env->mstatus = mstatus;
         break;