The Design and Implementation of the Anykernel and Rump Kernels

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3.2.3 The Hypercall Interface
The hypercall library implements the hypercall interface for the particular host. On
a POSIX host, the library is called librumpuser and the source code can be found
from lib/librumpuser. For historical reasons, the interface itself is also called
rumpuser, although for example rumphyper would be a more descriptive name.
As an example of a hypercall implementation we consider allocating“physical”memory
from the host. This hypercall is implemented by the hypercall library with a
call to posix_memalign() 4 . Conversely, the hypercall for releasing memory back
to the system is implemented with a call to free().
Theoretically, a rump kernel can accomplish this by calling the host directly. However,
there are a number of reasons a separate hypercall interface is better.
1. The user namespace is not available in the kernel. To for example make
the posix_memalign() call, the rump kernel has to define the prototype for
the function itself. If the prototype matches what libc expects, the call is
correctly resolved at link time. The downside is that duplicated information
can always go out-of-date.
Furthermore, calling out of the kernel is not
directly possible due to the symbol renaming we presented in Section 3.2.1.
2. A separate interface helps when hosting a rump kernel on a platform which
is not native to the rump kernel version.
For example, while all POSIX
platforms provide stat(const char *path, struct stat *sb), there is
no standard for the binary representation of struct stat.
Therefore, a
reference to the structure cannot be passed over the interface as binary data,
4 posix_memalign() is essentially malloc(), but it takes an alignment parameter in addition
to the size parameter. Kernel code assumes that allocating a page of memory will return it from
a page-aligned offset, and using posix_memalign() instead of malloc() allows to guarantee that
memory allocated by the hypercall will be page-aligned.