RISC-V: Workaround for critical mstatus.FS bug

This change is a workaround for a bug where mstatus.FS
is not correctly reporting dirty after operations that
modify floating point registers. This a critical bug
or RISC-V in QEMU as it results in floating point
register file corruption when running SMP Linux due to
task migration and possibly uniprocessor Linux if
more than one process is using the FPU.

This workaround will return dirty if mstatus.FS is
switched from off to initial or clean. According to
the specification it is legal for an implementation
to return only off, or dirty.

Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Alex Bennée <alex.bennee@linaro.org>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Michael Clark <mjc@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

diff --git a/target/riscv/op_helper.c b/target/riscv/op_helper.c
index e34715df4ec370688761c19bf5ace8caa20d1a4a..7c6068bac9584980c49e9a5d1cba594132f95f4c 100644 (file)
--- a/target/riscv/op_helper.c
+++ b/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;