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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: 7e7494627f ("s390x/virtio-ccw: Adapter interrupt support.")
Reported-by: Andre Wild <Andre.Wild1@ibm.com>
Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20200616045035.51641-2-pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
This commit is contained in:
Halil Pasic 2020-06-16 06:50:34 +02:00 committed by Cornelia Huck
parent 9bf728a09b
commit 1a8242f7c3
1 changed files with 10 additions and 8 deletions
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18
hw/s390x/virtio-ccw.c
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@ -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)