From: Halil Pasic <pasic@linux.ibm.com>
To: Matthew Rosato <mjrosato@linux.ibm.com>,
Cornelia Huck <cohuck@redhat.com>,
Christian Borntraeger <borntraeger@de.ibm.com>,
"Michael S. Tsirkin" <mst@redhat.com>,
qemu-s390x@nongnu.org, qemu-devel@nongnu.org
Cc: Halil Pasic <pasic@linux.ibm.com>, Thomas Huth <thuth@redhat.com>,
David Hildenbrand <david@redhat.com>,
Richard Henderson <rth@twiddle.net>
Subject: [PATCH 1/2] virtio-ccw: fix virtio_set_ind_atomic
Date: Tue, 16 Jun 2020 06:50:34 +0200 [thread overview]
Message-ID: <20200616045035.51641-2-pasic@linux.ibm.com> (raw)
In-Reply-To: <20200616045035.51641-1-pasic@linux.ibm.com>
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.
Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Reported-by: Andre Wild <Andre.Wild1@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;
}
static void virtio_ccw_notify(DeviceState *d, uint16_t vector)
--
2.17.1
next prev parent reply other threads:[~2020-06-16 4:51 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-16 4:50 [PATCH 0/2] two atomic_cmpxchg() related fixes Halil Pasic
2020-06-16 4:50 ` Halil Pasic [this message]
2020-06-16 5:58 ` [PATCH 1/2] virtio-ccw: fix virtio_set_ind_atomic Christian Borntraeger
2020-06-16 6:33 ` Cornelia Huck
2020-06-16 6:45 ` Christian Borntraeger
2020-06-19 7:14 ` Cornelia Huck
2020-06-17 23:56 ` Halil Pasic
2020-06-19 7:33 ` David Hildenbrand
2020-06-19 8:17 ` Halil Pasic
2020-06-16 9:31 ` Halil Pasic
2020-07-01 13:13 ` Christian Borntraeger
2020-07-04 18:34 ` Michael S. Tsirkin
2020-07-06 5:44 ` Christian Borntraeger
2020-07-06 11:19 ` Halil Pasic
2020-06-16 4:50 ` [PATCH 2/2] s390x/pci: fix set_ind_atomic Halil Pasic
2020-07-01 13:14 ` Christian Borntraeger
2020-07-01 12:01 ` [PATCH 0/2] two atomic_cmpxchg() related fixes Cornelia Huck
2020-07-01 12:06 ` Christian Borntraeger
2020-07-01 13:10 ` Cornelia Huck
2020-07-03 13:37 ` Halil Pasic
2020-07-03 14:03 ` Cornelia Huck
2020-07-02 11:18 ` Cornelia Huck
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