[libvirt] [PATCH v2 3/3] qemu: add memfd source type

[Michal Privoznik]

On 09/17/2018 03:14 PM, marcandre.lureau at redhat.com wrote:
> From: Marc-André Lureau <marcandre.lureau at redhat.com>
> 
> Add a new memoryBacking source type "memfd", supported by QEMU (when
> the apability is available).
> 
> A memfd is a specialized anonymous memory kind. As such, an anonymous
> source type could be automatically using a memfd. However, there are
> some complications when migrating from different memory backends in
> qemu (mainly due to the internal object naming at this point, but
> there could be more). For now, it is simpler and safer to simply
> introduce a new source type "memfd". Eventually, the "anonymous" type
> could learn to use memfd transparently in a seperate change.
> 
> The main benefits are that it doesn't need to create filesystem files,
> and it also enforces sealing, providing a bit more safety.
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau at redhat.com>
> ---
>  docs/formatdomain.html.in                     |  9 +--
>  docs/schemas/domaincommon.rng                 |  1 +
>  src/conf/domain_conf.c                        |  3 +-
>  src/conf/domain_conf.h                        |  1 +
>  src/qemu/qemu_command.c                       | 69 +++++++++++++------
>  src/qemu/qemu_domain.c                        | 12 +++-
>  .../memfd-memory-numa.x86_64-latest.args      | 34 +++++++++
>  tests/qemuxml2argvdata/memfd-memory-numa.xml  | 36 ++++++++++
>  tests/qemuxml2argvtest.c                      |  2 +
>  9 files changed, 140 insertions(+), 27 deletions(-)
>  create mode 100644 tests/qemuxml2argvdata/memfd-memory-numa.x86_64-latest.args
>  create mode 100644 tests/qemuxml2argvdata/memfd-memory-numa.xml
> 
> diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
> index 1f12ab5b42..eeee1f6d40 100644
> --- a/docs/formatdomain.html.in
> +++ b/docs/formatdomain.html.in
> @@ -1099,7 +1099,7 @@
>      </hugepages>
>      <nosharepages/>
>      <locked/>
> -    <source type="file|anonymous"/>
> +    <source type="file|anonymous|memfd"/>

I'm sorry but I do not think this is the way we should go. This
effectively avoids libvirt making the decision and exposes the backend
used directly. This puts unnecessary burden on mgmt applications because
they have to make yet another decision (track another domain attribute).

IIUC, memfd is like memory-backend-file and -ram combined. It can do
hugepages or just plain malloc(). Therefore it should be our first
choice for freshly started domains. And only if qemu doesn't support it
we should fall back to either -file or -ram backends.

This means we have to track what backend the domain was started with so
that we preserve that on migration (although, the fact that these
backends are not interchangeable makes me question 'backend' in their
name :-P). For that we can use status/migration XML as I suggested earlier.

Once again, status XML is not editable by user [*] and is used solely by
libvirtd to store runtime information for a running domain (and backend
used falls into that category).

Michal

* - sure, an evil admin could edit the status XML file (which is usually
stored under /var/run/libvirt/qemu/$domain.xml) and restart libvirtd to
reload the changes. But hey, the file is readable/writable by root only
and there are plenty other ways how an evil root could mess up with
running domains. We (have to) trust root.