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| -rw-r--r-- | developer-manual.adoc | 25 |
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diff --git a/developer-manual.adoc b/developer-manual.adoc index a538437..b7eb2cb 100644 --- a/developer-manual.adoc +++ b/developer-manual.adoc @@ -194,6 +194,31 @@ compile the policy files into the crosvm binary so paths don't have to be hardcoded. +==== The memfd server + +Spectrum's crosvm has been augmented with a feature called the "`memfd +server`" (currently only on the +https://spectrum-os.org/git/crosvm/log/?h=interguest[interguest] +branch). This is a small server that listens on a Unix stream socket. +It receives a request consisting of a name and a size, uses those to +allocate a +https://man7.org/linux/man-pages/man2/memfd_create.2.html[memfd], and +then sends the resulting memfd file descriptor back over the socket in +response, along with a single byte status code. + +The purpose of this is that it can be exposed to the guest over +virtio_wl. This means that the guest can request and receive +allocations of host memory. This is important, because virtio_wl only +allows file descriptors pointing to host memory to be sent over +virtio_wl -- a memfd allocated in a guest cannot be sent over virtio_wl. +It is rare that a guest needs to be able to allocate and send shared +memory in this way, but it is important for a Wayland compositor +running in a guest to be able to do this. + +The memfd server is disabled by default, but is enabled at startup +with the `--wl-memfd` flag. + + === Sommelier https://chromium.googlesource.com/chromiumos/platform2/+/master/vm_tools/sommelier[Sommelier] |