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xen-devel
Re: [Xen-devel] Automated para-virtualization
Hi Anthony,
On Apr 6, 2005, at 05:56, Anthony Liguori wrote:
Very interesting...
Thanks for the good questions and interest.
The basis of our solution is instruction substitution at the
assembler level
in order to replace the virtualization-sensitive operations of the
guest OS.
The virtualization-sensitive operations include instructions and
memory
accesses (such as to page tables or device registers).
So you annotate access to mmio or page tables? Was this for
performance or was it not possible to emulate mmio operations and trap
writes to the page table?
We annotate both, but in different annotation domains. We apply the
annotations for performance, especially the mmio annotations. The page
table references aren't as critical in many work loads, but if page
table activity is expected, then it is probably a necessary
optimization.
If you were able to avoid having to annotate these things, I presume
you could virtualize Linux with no modifications? Even if this
resulted in performance degradation, I can imagine scenarios where
having this option would be very useful (especially for supporting
legacy distributions).
This is true. It would be possible to avoid the annotations, and to
instead rely on traps. Although we prefer to automate these
annotations, which is currently our active work. And additionally, it
is possible to avoid heavy trapping on the page tables by tracking page
table accesses with the reference bits.
annotations). There are a few additional changes for the build
process.
Would it be possible to package your tools as a cross-compiler
environment so that all you had to do is set CROSS_COMPILE
appropriately?
Probably ... we don't handle 16-bit code right now, so the
CROSS_COMPILE solution needs a little work to avoid annotating the
16-bit code. The implication is that the current solution jumps to the
32-bit entry point, and relies on the runtime module preparing
"physical" memory with all the guest-specific boot loader information
(I guess that this is the "start of day" in Xen terminology).
Our current research is to enable run-time migrations between
incompatible
hypervisors, or between different versions of the same hypervisor, by
rewriting the instruction substitutions at time of migration.
Additionally,
we envisage that one can install a hypervisor underneath an OS which
runs on
bare metal.
Does this mean that you maintain the patch table and support
unpatching a patched image or do you simply keep a copy of the
unpatched kernel?
Either solution works ... just an implementation (or user
interface/management) issue.
Any thoughts on supporting kernel modules? Would you have to prepatch
a module?
Yes, modules must be supported long term. Modules need to be
annotated, and then patched at load time, thus probably requiring
collaboration from the OS to announce the installation of a new module.
The implication is that 3rd party modules benefit from automated
para-virtualization.
Have you implemented any other emulated devices?
Basic platform devices. We're working on high performance disk, and
the IO-APIC. Port I/O is particularly nice and easy, especially for
ports <= 0xff, and thus the XT-PIC emulation is subject to nice
rewriting.
Great! Look forward to seeing the code. Looks like you guys have
been doing really cool stuff :-)
Thanks,
Josh
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