On Thu, 21 Jan 2021 12:05:35 +0100
David Hildenbrand <da...@redhat.com> wrote:
> 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(+)
Acked-by: Alex Williamson <alex.william...@redhat.com
Reviewed-by: Alex Williamson <alex.william...@redhat.com>
> 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,
