spapr_pci: Fix number of returned vectors in ibm, change-msi

Current guest kernels try allocating as many vectors as the quota is.
For example, in the case of virtio-net (which has just 3 vectors)
the guest requests 4 vectors (that is the quota in the test) and
the existing ibm,change-msi handler returns 4. But before it returns,
it calls msix_set_message() in a loop and corrupts memory behind
the end of msix_table.

This limits the number of vectors returned by ibm,change-msi to
the maximum supported by the actual device.
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Cc: qemu-stable@nongnu.org
[agraf: squash in bugfix from aik]
Signed-off-by: NAlexander Graf <agraf@suse.de>

(cherry picked from commit b26696b5)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

hw/ppc/spapr_pci.c

@@ -343,6 +343,21 @@ static void rtas_ibm_change_msi(PowerPCCPU *cpu, sPAPREnvironment *spapr,
 
     /* There is no cached config, allocate MSIs */
     if (!phb->msi_table[ndev].nvec) {
+        int max_irqs = 0;
+        if (ret_intr_type == RTAS_TYPE_MSI) {
+            max_irqs = msi_nr_vectors_allocated(pdev);
+        } else if (ret_intr_type == RTAS_TYPE_MSIX) {
+            max_irqs = pdev->msix_entries_nr;
+        }
+        if (!max_irqs) {
+            error_report("Requested interrupt type %d is not enabled for device#%d",
+                         ret_intr_type, ndev);
+            rtas_st(rets, 0, -1); /* Hardware error */
+            return;
+        }
+        if (req_num > max_irqs) {
+            req_num = max_irqs;
+        }
         irq = spapr_allocate_irq_block(req_num, false,
                                        ret_intr_type == RTAS_TYPE_MSI);
         if (irq < 0) {