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RISC-V: Workaround for critical mstatus.FS bug

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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>
This commit is contained in:
Michael Clark 2018-03-23 16:49:07 -07:00
parent 47d3b60858
commit b02403363f
No known key found for this signature in database
GPG key ID: 6BF1D7B357EF3E4F
1 changed files with 15 additions and 2 deletions
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17
target/riscv/op_helper.c
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@ -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;