rendezvousing all physical CPUs

["Jan Beulich" <jbeulich@novell.com>]

Recently I created a Linux patch (in 2.6.18.3 and 2.6.19) that fixes problem
with the detection of the Firmware Hub based random number generator in
Intel chipsets. Little later I considered the consequences in Xen environments:
The patch needs to disable normal BIOS space in order to 'see through' to the
Firmware Hub interface. In native Linux, the stop_machine abstraction is
a very good mechanism to carry out this kind of job, but under Xen this isn't
strong enough, as it affects only those CPUs Dom0 is running on.

Will it be acceptable to create hypercall sub-functions (would probably go
into the platform group, but should be architecture independent) to allow
Dom0 to halt all physical CPUs but the current one, and later restart them?
Or should it rather be a single call with an event-channel based call back
to carry out the operation that must be protected?

Without such functionality, we'd have to disable the Intel RNG in the kernel,
or at least make it ignore any potentially available RNG unless told by the
user that it is safe to use *without* the detection sequence outlined by
Intel's documentation (which btw. has been found to not work when the
BIOS makes use of certain functionality that prevents access to the FWH
space).

Jan