Although RamDiscardMgr 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.
Cc: Paolo Bonzini <pbonz...@redhat.com>
Cc: "Michael S. Tsirkin" <m...@redhat.com>
Cc: Alex Williamson <alex.william...@redhat.com>
Cc: Dr. David Alan Gilbert <dgilb...@redhat.com>
Cc: Igor Mammedov <imamm...@redhat.com>
Cc: Pankaj Gupta <pankaj.gupta.li...@gmail.com>
Cc: Peter Xu <pet...@redhat.com>
Cc: Auger Eric <eric.au...@redhat.com>
Cc: Wei Yang <richard.weiy...@linux.alibaba.com>
Cc: teawater <teawat...@linux.alibaba.com>
Cc: Marek Kedzierski <mkedz...@redhat.com>
Signed-off-by: David Hildenbrand <da...@redhat.com>
---
hw/vfio/common.c | 43 +++++++++++++++++++++++++++++++++++++++++++
1 file changed, 43 insertions(+)
diff --git a/hw/vfio/common.c b/hw/vfio/common.c
index 78be813a53..166ec6ec62 100644
--- a/hw/vfio/common.c
+++ b/hw/vfio/common.c
@@ -761,6 +761,49 @@ static void
vfio_register_ram_discard_notifier(VFIOContainer *container,
vfio_ram_discard_notify_discard_all);
rdmc->register_listener(rdm, section->mr, &vrdl->listener);
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 RamDiscardMgr
+ * section consumes exactly one DMA mapping, with the exception of
+ * RamDiscardMgr sections; i.e., we don't expect to have gIOMMU sections in
+ * the same address space as RamDiscardMgr 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,
--
2.29.2
