target/arm: Store FPSR cumulative exception bits in env->vfp.fpsr

target/arm: Store FPSR cumulative exception bits in env->vfp.fpsr

Currently we store the FPSR cumulative exception bits in the
float_status fields, and use env->vfp.fpsr only for the NZCV bits.
(The QC bit is stored in env->vfp.qc[].)

This works for TCG, but if QEMU was built without CONFIG_TCG (i.e.
with KVM support only) then we use the stub versions of
vfp_get_fpsr_from_host() and vfp_set_fpsr_to_host() which do nothing,
throwing away the cumulative exception bit state.  The effect is that
if the FPSR state is round-tripped from KVM to QEMU then we lose the
cumulative exception bits.  In particular, this will happen if the VM
is migrated.  There is no user-visible bug when using KVM with a QEMU
binary that was built with CONFIG_TCG.

Fix this by always storing the cumulative exception bits in
env->vfp.fpsr.  If we are using TCG then we may also keep pending
cumulative exception information in the float_status fields, so we
continue to fold that in on reads.

This change will also be helpful for implementing FEAT_AFP later,
because that includes a feature where in some situations we want to
cause input denormals to be flushed to zero without affecting the
existing state of the FPSR.IDC bit, so we need a place to store IDC
which is distinct from the various float_status fields.

(Note for stable backports: the bug goes back to 4a15527c9fee but
this code was refactored in commits ea8618382aba..a8ab8706d4cc461, so
fixing it in branches without those refactorings will mean either
backporting the refactor or else implementing a conceptually similar
fix for the old code.)

Cc: qemu-stable@nongnu.org
Fixes: 4a15527c9fee ("target/arm/vfp_helper: Restrict the SoftFloat use to TCG")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241011162401.3672735-1-peter.maydell@linaro.org
(cherry picked from commit d9c7adb6019f2ac3d6a5a36c4121341f4b6424af)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>