migration/multifd: Don't send device state packets with zerocopy flag

If zerocopy is enabled for multifd then QIO_CHANNEL_WRITE_FLAG_ZERO_COPY
flag is forced into all multifd channel write calls via p->write_flags
that was setup in multifd_nocomp_send_setup().

However, device state packets aren't compatible with zerocopy - the data
buffer isn't getting kept pinned until multifd channel flush.

Make sure to mask that QIO_CHANNEL_WRITE_FLAG_ZERO_COPY flag in a multifd
send thread if the data being sent is device state.

Fixes: 0525b91a0b99 ("migration/multifd: Device state transfer support - send side")
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/3bd5f48578e29f3a78f41b1e4fbea3d4b2d9b136.1747403393.git.maciej.szmigiero@oracle.com
Signed-off-by: Peter Xu <peterx@redhat.com>
(cherry picked from commit 6be7696129b302830a9cff7e30484e08c2d64b57)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

migration/multifd.c

@@ -695,6 +695,7 @@ static void *multifd_send_thread(void *opaque)
         if (qatomic_load_acquire(&p->pending_job)) {
             bool is_device_state = multifd_payload_device_state(p->data);
             size_t total_size;
+            int write_flags_masked = 0;
 
             p->flags = 0;
             p->iovs_num = 0;
@@ -702,6 +703,9 @@ static void *multifd_send_thread(void *opaque)
 
             if (is_device_state) {
                 multifd_device_state_send_prepare(p);
+
+                /* Device state packets cannot be sent via zerocopy */
+                write_flags_masked |= QIO_CHANNEL_WRITE_FLAG_ZERO_COPY;
             } else {
                 ret = multifd_send_state->ops->send_prepare(p, &local_err);
                 if (ret != 0) {
@@ -723,7 +727,8 @@ static void *multifd_send_thread(void *opaque)
                                               &p->data->u.ram, &local_err);
             } else {
                 ret = qio_channel_writev_full_all(p->c, p->iov, p->iovs_num,
-                                                  NULL, 0, p->write_flags,
+                                                  NULL, 0,
+                                                  p->write_flags & ~write_flags_masked,
                                                   &local_err);
             }