From qapi-schema.json:

# @vectors: #optional number of MSI-x vectors, 0 to disable MSI-X


When hotplug a virtio-pci device, qemu will try to take over PCI BARs for the use of MSIX,
qemu only remains 4k memory for BAR & vector table, each msix entry takes 16 bytes. So it
only allows 4k /16 bytes = 128 MSIX.

When we assign a vectors that is larger than 128, the msix resource isn't enough, so msix
will be disabled.

So it's not a bug.


Related commit:
========================================================================
commit 53f949254ad2435bfd45cb0dee96f246a0bdd7e3
Author: Alex Williamson <alex.williamson>
Date:   Thu Jun 14 12:15:51 2012 -0600

    msix: Add simple BAR allocation MSIX setup functions
    
    msi_init() takes over a BAR without really specifying or allowing
    specification of how it does so.  Instead, let's split it into
    two interfaces, one fully specified, and one trivially easy.  This
    implements the latter.  msix_init_exclusive_bar() takes over
    allocating and filling a PCI BAR _exclusively_ for the use of MSIX.
    When used, the matching msi_uninit_exclusive_bar() should be used
    to tear it down.


+int msix_init_exclusive_bar(PCIDevice *dev, unsigned short nentries,
+                            uint8_t bar_nr)
+{
+    int ret;
+    char *name;
+
+    /*
+     * Migration compatibility dictates that this remains a 4k
+     * BAR with the vector table in the lower half and PBA in
+     * the upper half.  Do not use these elsewhere!
+     */
+#define MSIX_EXCLUSIVE_BAR_SIZE 4096
+#define MSIX_EXCLUSIVE_BAR_PBA_OFFSET (MSIX_EXCLUSIVE_BAR_SIZE / 2)
+
+    if (nentries * PCI_MSIX_ENTRY_SIZE > MSIX_EXCLUSIVE_BAR_PBA_OFFSET) {
+        return -EINVAL;
+    }

......

It's better to report an error when fail to init exclusive bars for msix.

[Qemu-devel] [PATCH] virtio-pci: report an error when disable msix
http://lists.nongnu.org/archive/html/qemu-devel/2014-05/msg04475.html

Upstream applied another similar patch from Fam, backport is needed.

[PATCH] virtio-pci: Report an error when msix vectors init fails

commit c7ff54825b74f27c3aac85aad540542801630d0a
Author: Fam Zheng <famz>
Date:   Mon Jun 23 17:06:25 2014 +0800

    virtio-pci: Report an error when msix vectors init fails
    
    Currently vectors silently cleared to 0 if the initialization is failed,
    but user should at least have one way to notice this.
    
    Signed-off-by: Fam Zheng <famz>
    Reviewed-by: Michael S. Tsirkin <mst>
    Signed-off-by: Michael S. Tsirkin <mst>

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index ce97514..57e1e61 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -976,6 +976,8 @@ static void virtio_pci_device_plugged(DeviceState *d)
 
     if (proxy->nvectors &&
         msix_init_exclusive_bar(&proxy->pci_dev, proxy->nvectors, 1)) {
+        error_report("unable to init msix vectors to %" PRIu32,
+                     proxy->nvectors);
         proxy->nvectors = 0;
     }

There are two bugs, they were found with different devices, but it's duplicated issue.

*** This bug has been marked as a duplicate of bug 1095645 ***