vfio: Sanity check maximum number of DMA mappings with RamDiscardManager

Although RamDiscardManager can handle running into the maximum number of
DMA mappings by propagating errors when creating a DMA mapping, we want
to sanity check and warn the user early that there is a theoretical setup
issue and that virtio-mem might not be able to provide as much memory
towards a VM as desired.

As suggested by Alex, let's use the number of KVM memory slots to guess
how many other mappings we might see over time.

Acked-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Auger Eric <eric.auger@redhat.com>
Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
Cc: teawater <teawaterz@linux.alibaba.com>
Cc: Marek Kedzierski <mkedzier@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210413095531.25603-9-david@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

1 files changed, 43 insertions, 0 deletions

diff --git a/hw/vfio/common.c b/hw/vfio/common.c
index 79628d60ae..f8a2fe8441 100644
--- a/hw/vfio/common.c
+++ b/hw/vfio/common.c
@@ -728,6 +728,49 @@ static void vfio_register_ram_discard_listener(VFIOContainer *container,
                               vfio_ram_discard_notify_discard, true);
     ram_discard_manager_register_listener(rdm, &vrdl->listener, section);
     QLIST_INSERT_HEAD(&container->vrdl_list, vrdl, next);
+
+    /*
+     * Sanity-check if we have a theoretically problematic setup where we could
+     * exceed the maximum number of possible DMA mappings over time. We assume
+     * that each mapped section in the same address space as a RamDiscardManager
+     * section consumes exactly one DMA mapping, with the exception of
+     * RamDiscardManager sections; i.e., we don't expect to have gIOMMU sections
+     * in the same address space as RamDiscardManager sections.
+     *
+     * We assume that each section in the address space consumes one memslot.
+     * We take the number of KVM memory slots as a best guess for the maximum
+     * number of sections in the address space we could have over time,
+     * also consuming DMA mappings.
+     */
+    if (container->dma_max_mappings) {
+        unsigned int vrdl_count = 0, vrdl_mappings = 0, max_memslots = 512;
+
+#ifdef CONFIG_KVM
+        if (kvm_enabled()) {
+            max_memslots = kvm_get_max_memslots();
+        }
+#endif
+
+        QLIST_FOREACH(vrdl, &container->vrdl_list, next) {
+            hwaddr start, end;
+
+            start = QEMU_ALIGN_DOWN(vrdl->offset_within_address_space,
+                                    vrdl->granularity);
+            end = ROUND_UP(vrdl->offset_within_address_space + vrdl->size,
+                           vrdl->granularity);
+            vrdl_mappings += (end - start) / vrdl->granularity;
+            vrdl_count++;
+        }
+
+        if (vrdl_mappings + max_memslots - vrdl_count >
+            container->dma_max_mappings) {
+            warn_report("%s: possibly running out of DMA mappings. E.g., try"
+                        " increasing the 'block-size' of virtio-mem devies."
+                        " Maximum possible DMA mappings: %d, Maximum possible"
+                        " memslots: %d", __func__, container->dma_max_mappings,
+                        max_memslots);
+        }
+    }
 }
 
 static void vfio_unregister_ram_discard_listener(VFIOContainer *container,