Re: [PATCH v4 11/11] vfio: Disable only uncoordinated discards

[Alex Williamson <alex.williamson@redhat.com>]

On Thu,  7 Jan 2021 14:34:23 +0100
David Hildenbrand <david@redhat.com> wrote:

> We support coordinated discarding of RAM using the RamDiscardMgr. Let's
> unlock support for coordinated discards, keeping uncoordinated discards
> (e.g., via virtio-balloon) disabled.
> 
> This unlocks virtio-mem + vfio. Note that vfio used via "nvme://" by the
> block layer has to be implemented/unlocked separately. For now,
> virtio-mem only supports x86-64 - spapr IOMMUs are not tested/affected.

I think SPAPR always uses a vIOMMU, but I won't claim to understand it.
Is there anything other than testing that restricts it to x86-64?  ARM
and S390 will use the same type1 IOMMU backend as far as vfio support
is concerned.  Should we do something more than put it in a commit log
if we really want to prevent it elsewhere?  Thanks,

Alex
 
> Note: The block size of a virtio-mem device has to be set to sane sizes,
> depending on the maximum hotplug size - to not run out of vfio mappings.
> The default virtio-mem block size is usually in the range of a couple of
> MBs. The maximum number of mapping is 64k, shared with other users.
> Assume you want to hotplug 256GB using virtio-mem - the block size would
> have to be set to at least 8 MiB (resulting in 32768 separate mappings).
> 
> Reviewed-by: Pankaj Gupta <pankaj.gupta@cloud.ionos.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>
> ---
>  hw/vfio/common.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> index 2b9ba3f8c9..ee7a82524a 100644
> --- a/hw/vfio/common.c
> +++ b/hw/vfio/common.c
> @@ -1976,8 +1976,10 @@ static int vfio_connect_container(VFIOGroup *group, AddressSpace *as,
>       * new memory, it will not yet set ram_block_discard_set_required() and
>       * therefore, neither stops us here or deals with the sudden memory
>       * consumption of inflated memory.
> +     *
> +     * We do support discarding of memory coordinated via the RamDiscardMgr.
>       */
> -    ret = ram_block_discard_disable(true);
> +    ret = ram_block_uncoordinated_discard_disable(true);
>      if (ret) {
>          error_setg_errno(errp, -ret, "Cannot set discarding of RAM broken");
>          return ret;
> @@ -2157,7 +2159,7 @@ close_fd_exit:
>      close(fd);
>  
>  put_space_exit:
> -    ram_block_discard_disable(false);
> +    ram_block_uncoordinated_discard_disable(false);
>      vfio_put_address_space(space);
>  
>      return ret;
> @@ -2279,7 +2281,7 @@ void vfio_put_group(VFIOGroup *group)
>      }
>  
>      if (!group->ram_block_discard_allowed) {
> -        ram_block_discard_disable(false);
> +        ram_block_uncoordinated_discard_disable(false);
>      }
>      vfio_kvm_device_del_group(group);
>      vfio_disconnect_container(group);
> @@ -2333,7 +2335,7 @@ int vfio_get_device(VFIOGroup *group, const char *name,
>  
>          if (!group->ram_block_discard_allowed) {
>              group->ram_block_discard_allowed = true;
> -            ram_block_discard_disable(false);
> +            ram_block_uncoordinated_discard_disable(false);
>          }
>      }
>