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[Qemu-devel] [PATCH v2 27/42] ivshmem: Receive shared memory synchronous
From: |
Markus Armbruster |
Subject: |
[Qemu-devel] [PATCH v2 27/42] ivshmem: Receive shared memory synchronously in realize() |
Date: |
Mon, 7 Mar 2016 20:25:39 +0100 |
When configured for interrupts (property "chardev" given), we receive
the shared memory from an ivshmem server. We do so asynchronously
after realize() completes, by setting up callbacks with
qemu_chr_add_handlers().
Keeping server I/O out of realize() that way avoids delays due to a
slow server. This is probably relevant only for hot plug.
However, this funny "no shared memory, yet" state of the device also
causes a raft of issues that are hard or impossible to work around:
* The guest is exposed to this state: when we enter and leave it its
shared memory contents is apruptly replaced, and device register
IVPosition changes.
This is a known issue. We document that guests should not access
the shared memory after device initialization until the IVPosition
register becomes non-negative.
For cold plug, the funny state is unlikely to be visible in
practice, because we normally receive the shared memory long before
the guest gets around to mess with the device.
For hot plug, the timing is tighter, but the relative slowness of
PCI device configuration has a good chance to hide the funny state.
In either case, guests complying with the documented procedure are
safe.
* Migration becomes racy.
If migration completes before the shared memory setup completes on
the source, shared memory contents is silently lost. Fortunately,
migration is rather unlikely to win this race.
If the shared memory's ramblock arrives at the destination before
shared memory setup completes, migration fails.
There is no known way for a management application to wait for
shared memory setup to complete.
All you can do is retry failed migration. You can improve your
chances by leaving more time between running the destination QEMU
and the migrate command.
To mitigate silent memory loss, you need to ensure the server
initializes shared memory exactly the same on source and
destination.
These issues are entirely undocumented so far.
I'd expect the server to be almost always fast enough to hide these
issues. But then rare catastrophic races are in a way the worst kind.
This is way more trouble than I'm willing to take from any device.
Kill the funny state by receiving shared memory synchronously in
realize(). If your hot plug hangs, go kill your ivshmem server.
For easier review, this commit only makes the receive synchronous, it
doesn't add the necessary error propagation. Without that, the funny
state persists. The next commit will do that, and kill it off for
real.
Signed-off-by: Markus Armbruster <address@hidden>
---
hw/misc/ivshmem.c | 68 ++++++++++++++++++++++++++++++++++++----------------
tests/ivshmem-test.c | 26 ++++++--------------
2 files changed, 55 insertions(+), 39 deletions(-)
diff --git a/hw/misc/ivshmem.c b/hw/misc/ivshmem.c
index 2f2f43f..24da19e 100644
--- a/hw/misc/ivshmem.c
+++ b/hw/misc/ivshmem.c
@@ -675,27 +675,45 @@ static void ivshmem_read(void *opaque, const uint8_t
*buf, int size)
process_msg(s, msg, fd);
}
-static void ivshmem_check_version(void *opaque, const uint8_t * buf, int size)
+static int64_t ivshmem_recv_msg(IVShmemState *s, int *pfd)
{
- IVShmemState *s = opaque;
- int tmp;
- int64_t version;
+ int64_t msg;
+ int n, ret;
- if (!fifo_update_and_get_i64(s, buf, size, &version)) {
- return;
- }
+ n = 0;
+ do {
+ ret = qemu_chr_fe_read_all(s->server_chr, (uint8_t *)&msg + n,
+ sizeof(msg) - n);
+ if (ret < 0 && ret != -EINTR) {
+ /* TODO error handling */
+ return INT64_MIN;
+ }
+ n += ret;
+ } while (n < sizeof(msg));
- tmp = qemu_chr_fe_get_msgfd(s->server_chr);
- if (tmp != -1 || version != IVSHMEM_PROTOCOL_VERSION) {
+ *pfd = qemu_chr_fe_get_msgfd(s->server_chr);
+ return msg;
+}
+
+static void ivshmem_recv_setup(IVShmemState *s)
+{
+ int64_t msg;
+ int fd;
+
+ msg = ivshmem_recv_msg(s, &fd);
+ if (fd != -1 || msg != IVSHMEM_PROTOCOL_VERSION) {
fprintf(stderr, "incompatible version, you are connecting to a
ivshmem-"
"server using a different protocol please check your setup\n");
- qemu_chr_add_handlers(s->server_chr, NULL, NULL, NULL, s);
return;
}
- IVSHMEM_DPRINTF("version check ok, switch to real chardev handler\n");
- qemu_chr_add_handlers(s->server_chr, ivshmem_can_receive, ivshmem_read,
- NULL, s);
+ /*
+ * Receive more messages until we got shared memory.
+ */
+ do {
+ msg = ivshmem_recv_msg(s, &fd);
+ process_msg(s, msg, fd);
+ } while (msg != -1);
}
/* Select the MSI-X vectors used by device.
@@ -902,19 +920,29 @@ static void pci_ivshmem_realize(PCIDevice *dev, Error
**errp)
IVSHMEM_DPRINTF("using shared memory server (socket = %s)\n",
s->server_chr->filename);
- if (ivshmem_setup_interrupts(s) < 0) {
- error_setg(errp, "failed to initialize interrupts");
- return;
- }
-
/* we allocate enough space for 16 peers and grow as needed */
resize_peers(s, 16);
s->vm_id = -1;
pci_register_bar(dev, 2, attr, &s->bar);
- qemu_chr_add_handlers(s->server_chr, ivshmem_can_receive,
- ivshmem_check_version, NULL, s);
+ /*
+ * Receive setup messages from server synchronously.
+ * Older versions did it asynchronously, but that creates a
+ * number of entertaining race conditions.
+ * TODO Propagate errors! Without that, we still have races
+ * on errors.
+ */
+ ivshmem_recv_setup(s);
+ if (memory_region_is_mapped(&s->ivshmem)) {
+ qemu_chr_add_handlers(s->server_chr, ivshmem_can_receive,
+ ivshmem_read, NULL, s);
+ }
+
+ if (ivshmem_setup_interrupts(s) < 0) {
+ error_setg(errp, "failed to initialize interrupts");
+ return;
+ }
} else {
/* just map the file immediately, we're not using a server */
int fd;
diff --git a/tests/ivshmem-test.c b/tests/ivshmem-test.c
index 7b6b957..c7f3758 100644
--- a/tests/ivshmem-test.c
+++ b/tests/ivshmem-test.c
@@ -309,35 +309,23 @@ static void test_ivshmem_server(bool msi)
ret = ivshmem_server_start(&server);
g_assert_cmpint(ret, ==, 0);
- setup_vm_with_server(&state1, nvectors, msi);
- s1 = &state1;
- setup_vm_with_server(&state2, nvectors, msi);
- s2 = &state2;
-
- /* check state before server sends stuff */
- g_assert_cmpuint(in_reg(s1, IVPOSITION), ==, 0xffffffff);
- g_assert_cmpuint(in_reg(s2, IVPOSITION), ==, 0xffffffff);
- g_assert_cmpuint(qtest_readb(s1->qtest, (uintptr_t)s1->mem_base), ==,
0x00);
-
thread.server = &server;
ret = pipe(thread.pipe);
g_assert_cmpint(ret, ==, 0);
thread.thread = g_thread_new("ivshmem-server", server_thread, &thread);
g_assert(thread.thread != NULL);
- /* waiting for devices to become operational */
- while (g_get_monotonic_time() < end_time) {
- g_usleep(1000);
- if ((int)in_reg(s1, IVPOSITION) >= 0 &&
- (int)in_reg(s2, IVPOSITION) >= 0) {
- break;
- }
- }
+ setup_vm_with_server(&state1, nvectors, msi);
+ s1 = &state1;
+ setup_vm_with_server(&state2, nvectors, msi);
+ s2 = &state2;
/* check got different VM ids */
vm1 = in_reg(s1, IVPOSITION);
vm2 = in_reg(s2, IVPOSITION);
- g_assert_cmpuint(vm1, !=, vm2);
+ g_assert_cmpint(vm1, >=, 0);
+ g_assert_cmpint(vm2, >=, 0);
+ g_assert_cmpint(vm1, !=, vm2);
/* check number of MSI-X vectors */
global_qtest = s1->qtest;
--
2.4.3
- Re: [Qemu-devel] [PATCH v2 28/42] ivshmem: Propagate errors through ivshmem_recv_setup(), (continued)
[Qemu-devel] [PATCH v2 42/42] contrib/ivshmem-server: Print "not for production" warning, Markus Armbruster, 2016/03/07
[Qemu-devel] [PATCH v2 41/42] ivshmem: Require master to have ID zero, Markus Armbruster, 2016/03/07
[Qemu-devel] [PATCH v2 07/42] event_notifier: Make event_notifier_init_fd() #ifdef CONFIG_EVENTFD, Markus Armbruster, 2016/03/07
[Qemu-devel] [PATCH v2 22/42] ivshmem: Leave INTx alone when using MSI-X, Markus Armbruster, 2016/03/07
[Qemu-devel] [PATCH v2 15/42] ivshmem: Clean up after commit 9940c32, Markus Armbruster, 2016/03/07
[Qemu-devel] [PATCH v2 18/42] ivshmem: Fix harmless misuse of Error, Markus Armbruster, 2016/03/07
[Qemu-devel] [PATCH v2 27/42] ivshmem: Receive shared memory synchronously in realize(),
Markus Armbruster <=
[Qemu-devel] [PATCH v2 32/42] ivshmem: Tighten check of property "size", Markus Armbruster, 2016/03/07
[Qemu-devel] [PATCH v2 26/42] ivshmem: Plug leaks on unplug, fix peer disconnect, Markus Armbruster, 2016/03/07
[Qemu-devel] [PATCH v2 37/42] ivshmem: Replace int role_val by OnOffAuto master, Markus Armbruster, 2016/03/07
[Qemu-devel] [PATCH v2 39/42] ivshmem: Clean up after the previous commit, Markus Armbruster, 2016/03/07
[Qemu-devel] [PATCH v2 31/42] ivshmem: Simplify how we cope with short reads from server, Markus Armbruster, 2016/03/07
[Qemu-devel] [PATCH v2 29/42] ivshmem: Rely on server sending the ID right after the version, Markus Armbruster, 2016/03/07