Adding POSTCOPY support

[ShadowCurse]

Summary of the PR

Added ability to use VHOST_USER_POSTCOPY_ADVISE, VHOST_USER_POSTCOPY_LISTEN and VHOST_USER_POSTCOPY_END messages. This includes sending them from the frontend and processing them on the backend.

The POSTCOPY messages are only usable when
VHOST_USER_PROTOCOL_F_PAGEFAULT feature is negotiated.

Messages and their descriptions are:

• VHOST_USER_POSTCOPY_ADVISE:
  When the front-end sends this message to the backend,
  the back-end must open a userfaultfd for later use
  and send it's fd to the front-end.

• VHOST_USER_POSTCOPY_LISTEN
  When the back-end receives this message it must ensure
  that shared memory is registered with userfaultfd to
  cause faulting of non-present pages.
  This is always sent sometime after a VHOST_USER_POSTCOPY_ADVISE.

• VHOST_USER_POSTCOPY_END
  When the back-end receives this message it must disable the
  userfaultfd. The reply is an acknowledgement only.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR are signed (with git commit -s), and the commit
  message has max 60 characters for the summary and max 75 characters for each
  description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[Ablu]

I am not sure if this works with operation modes where regions are mapped on demand (Xen grants, for example). But I will let @vireshk comment on that.

Otherwise, I think commit messages are pretty bare-bone. Not everyone (at least not me :)) is an expert on these migration topics so it might help to explain what is happening in more detail.

[Ablu]

@germag: This may be related to your work on migration?

[ShadowCurse]

Hi @Ablu, I have addressed comments you left before, please take another look. Also @vireshk and @germag, would be nice to hear your thoughts as well.

Regarding ci not passing with musl toolchain, I am working on this.

[stefano-garzarella]

@ShadowCurse did you test the changes of rust-vmm-container locally?
You can use ./docker.sh build to build a local image and try to build this branch through it.

For example, what I usually do is:

cd rust-vmm-container
./docker.sh build

cd vhost
# I'm using podman, but with docker should be similar
podman run -it --rm --security-opt=seccomp=unconfined --volume $(pwd):/workdir --workdir /workdir localhost/rustvmm/dev:v33_x86_64 
cargo build --target x86_64-unknown-linux-musl

[Ablu]

/cc @stsquad

[Ablu]

The typo makes me a bit nervous. It probably results in a no-op for all the builds, or do I miss something here?

Could you elaborate how you tested this? Would it maybe make sense to add automated tests?

[stefano-garzarella]

Now that we have tests for the new code, maybe we can enable postcopy for coverage:

diff --git a/coverage_config_x86_64.json b/coverage_config_x86_64.json
index d201e56..9923e17 100644
--- a/coverage_config_x86_64.json
+++ b/coverage_config_x86_64.json
@@ -1,5 +1,5 @@
 {
   "coverage_score": 75.0,
   "exclude_path": "vhost/src/vhost_kern/",
-  "crate_features": "vhost/vhost-user-frontend,vhost/vhost-user-backend"
+  "crate_features": "vhost/vhost-user-frontend,vhost/vhost-user-backend,vhost-user-backend/postcopy"
 }

I tried it locally and the coverage test passes successfully.

[stefano-garzarella]

LGTM!

Nit: there is a little typo in the last commit title (s/coverade/coverage).

[Ablu]

Thanks!