On Sat, 4 Jul 2020 14:34:04 -0400
"Michael S. Tsirkin" <m...@redhat.com> wrote:
> On Tue, Jun 16, 2020 at 06:50:34AM +0200, Halil Pasic wrote:
> > The atomic_cmpxchg() loop is broken because we occasionally end up with
> > old and _old having different values (a legit compiler can generate code
> > that accessed *ind_addr again to pick up a value for _old instead of
> > using the value of old that was already fetched according to the
> > rules of the abstract machine). This means the underlying CS instruction
> > may use a different old (_old) than the one we intended to use if
> > atomic_cmpxchg() performed the xchg part.
>
> And was this ever observed in the field? Or is this a theoretical issue?
> commit log should probably say ...
>
It was observed in the field (Christian already answered). I think the
message already implies this, because the only conjunctive is about the
compiler behavior.
> >
> > Let us use volatile to force the rules of the abstract machine for
> > accesses to *ind_addr. Let us also rewrite the loop so, we that the
>
> we that -> we know that?
s/we//
It would be nice to fix this before the patch gets merged.
>
> > new old is used to compute the new desired value if the xchg part
> > is not performed.
> >
> > Signed-off-by: Halil Pasic <pa...@linux.ibm.com>
> > Reported-by: Andre Wild <andre.wi...@ibm.com>
> > Fixes: 7e7494627f ("s390x/virtio-ccw: Adapter interrupt support.")
> > ---
> > hw/s390x/virtio-ccw.c | 18 ++++++++++--------
> > 1 file changed, 10 insertions(+), 8 deletions(-)
> >
> > diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
> > index c1f4bb1d33..3c988a000b 100644
> > --- a/hw/s390x/virtio-ccw.c
> > +++ b/hw/s390x/virtio-ccw.c
> > @@ -786,9 +786,10 @@ static inline VirtioCcwDevice
> > *to_virtio_ccw_dev_fast(DeviceState *d)
> > static uint8_t virtio_set_ind_atomic(SubchDev *sch, uint64_t ind_loc,
> > uint8_t to_be_set)
> > {
> > - uint8_t ind_old, ind_new;
> > + uint8_t expected, actual;
> > hwaddr len = 1;
> > - uint8_t *ind_addr;
> > + /* avoid multiple fetches */
> > + uint8_t volatile *ind_addr;
> >
> > ind_addr = cpu_physical_memory_map(ind_loc, &len, true);
> > if (!ind_addr) {
> > @@ -796,14 +797,15 @@ static uint8_t virtio_set_ind_atomic(SubchDev *sch,
> > uint64_t ind_loc,
> > __func__, sch->cssid, sch->ssid, sch->schid);
> > return -1;
> > }
> > + actual = *ind_addr;
> > do {
> > - ind_old = *ind_addr;
> > - ind_new = ind_old | to_be_set;
> > - } while (atomic_cmpxchg(ind_addr, ind_old, ind_new) != ind_old);
> > - trace_virtio_ccw_set_ind(ind_loc, ind_old, ind_new);
> > - cpu_physical_memory_unmap(ind_addr, len, 1, len);
> > + expected = actual;
> > + actual = atomic_cmpxchg(ind_addr, expected, expected | to_be_set);
> > + } while (actual != expected);
> > + trace_virtio_ccw_set_ind(ind_loc, actual, actual | to_be_set);
> > + cpu_physical_memory_unmap((void *)ind_addr, len, 1, len);
> >
> > - return ind_old;
> > + return actual;
> > }
>
> I wonder whether cpuXX APIs should accept volatile pointers, too:
> casting away volatile is always suspicious.
> But that is a separate issue ...
>
Nod.
Thanks for having a look!
> > static void virtio_ccw_notify(DeviceState *d, uint16_t vector)
> > --
> > 2.17.1
>
