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QEMU keeps the s390x floating-point control register (FPC) in env->fpc.
The rounding mode bits [2:0] of FPC are reflected into the derived
env->fpu_status via set_float_rounding_mode(); every architectural
write to FPC goes through HELPER(sfpc) which keeps the two in sync.
restore_sigregs() restored FPC with a direct assignment:
__get_user(env->fpc, &sc->fpregs.fpc);
This wrote env->fpc correctly but never updated env->fpu_status, so on
sigreturn the interrupted code resumed with whatever rounding mode the
signal handler last installed in fpu_status.
Factor the two-step "write fpc + sync fpu_status" logic out of
HELPER(sfpc) into cpu_s390x_load_fpc(), declare it in cpu.h, and call
it from restore_sigregs() in place of the direct assignment.
cpu_s390x_load_fpc() partially reuses the sanity check from
HELPER(sfpc): if the FPC value has an invalid rounding mode or reserved
bits set, it falls back to 0, matching the kernel's fpu_lfpc_safe()
behavior where a corrupt signal frame value causes a specification
exception and 0 is used instead.
HELPER(sfpc) now calls cpu_s390x_load_fpc() after its full
specification-exception check, including the FEAT_FLOATING_POINT_EXT
test that is not needed for the signal restore path.
Fixes: 2941e0fa05 ("linux-user/s390x: Save/restore fpc when handling a signal")
Cc: qemu-stable@nongnu.org
Signed-off-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>