s390x/pci: fix 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: 8cba80c3a0 ("s390: Add PCI bus support")
Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20200616045035.51641-3-pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
diff --git a/hw/s390x/s390-pci-bus.c b/hw/s390x/s390-pci-bus.c
index 142e52a..736965c 100644
--- a/hw/s390x/s390-pci-bus.c
+++ b/hw/s390x/s390-pci-bus.c
@@ -637,22 +637,24 @@
static uint8_t set_ind_atomic(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) {
s390_pci_generate_error_event(ERR_EVENT_AIRERR, 0, 0, 0, 0);
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);
- cpu_physical_memory_unmap(ind_addr, len, 1, len);
+ expected = actual;
+ actual = atomic_cmpxchg(ind_addr, expected, expected | to_be_set);
+ } while (actual != expected);
+ cpu_physical_memory_unmap((void *)ind_addr, len, 1, len);
- return ind_old;
+ return actual;
}
static void s390_msi_ctrl_write(void *opaque, hwaddr addr, uint64_t data,
