From b02403363f1056421d120c8e974fdf9c76a84f95 Mon Sep 17 00:00:00 2001 From: Michael Clark Date: Fri, 23 Mar 2018 16:49:07 -0700 Subject: [PATCH] RISC-V: Workaround for critical mstatus.FS bug MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Cc: Sagar Karandikar Cc: Bastian Koppelmann Cc: Peter Maydell Cc: Alex Bennée Cc: Richard Henderson Cc: Philippe Mathieu-Daudé Tested-by: Richard W.M. Jones Signed-off-by: Michael Clark Reviewed-by: Richard Henderson --- target/riscv/op_helper.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-) diff --git a/target/riscv/op_helper.c b/target/riscv/op_helper.c index e34715df4e..7c6068bac9 100644 --- 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; -- 2.11.0