Commit 1a8242f7 authored by Halil Pasic's avatar Halil Pasic Committed by Cornelia Huck
Browse files

virtio-ccw: fix virtio_set_ind_atomic 



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.

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
new old is used to compute the new desired value if the xchg part
is not performed.

Fixes: 7e749462 ("s390x/virtio-ccw: Adapter interrupt support.")
Reported-by: default avatarAndre Wild <Andre.Wild1@ibm.com>
Signed-off-by: default avatarHalil Pasic <pasic@linux.ibm.com>
Reviewed-by: default avatarChristian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20200616045035.51641-2-pasic@linux.ibm.com>
Signed-off-by: default avatarCornelia Huck <cohuck@redhat.com>
parent 9bf728a0

hw/s390x/virtio-ccw.c

+10 −8
Original line number Diff line number Diff line
@@ -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;

}



static void virtio_ccw_notify(DeviceState *d, uint16_t vector)