Re: Porting of Guest OS

[David Hopwood <david.nospam.hopwood@blueyonder.co.uk>]

Ronald G. Minnich wrote:
> On Fri, 31 Dec 2004, David Hopwood wrote:
> 
>>It should be possible to make it work by using the hypervisor interface
>>equivalents of rdmsr/wrmsr. See section 9.11 of the IA32 architecture
>>manual Volume 3.

<http://developer.intel.com/design/pentium4/manuals/index_new.htm>
<ftp://download.intel.com/design/Pentium4/manuals/25366814.pdf>

> interesting. Part of the microcode upgrade process is pointing the CPU at 
> a chunk of memory that it ingests as the new microcode. I found the whole 
> business a bit dicey to think about from ring 1, but I guess it ought to 
> be fine.

I really shouldn't post at that time of the morning. First, it should have
been obvious that DOM0 only runs on a single CPU, since multiprocessor
guests aren't supported yet. Second, the architecture manual only gives
sample code for doing the update in real mode; it seems to imply that this
will also work from protected mode, but it's not quite clear what the
requirements are. Based on the code of the Linux kernel driver:

   <http://lxr.linux.no/source/arch/i386/kernel/microcode.c#L288>

it looks like EAX just has to contain a linear address when Linux is
running on bare hardware (the driver casts a pointer in the data segment,
which is zero-based, to an unsigned int). In order to use a generic
wrmsr hypercall, DOM0 would have to calculate what linear address Xen
should use, which although possible (I think), is a horrible layering
violation that would depend on implementation details of Xen.

It would be much cleaner to either:

  - add a privileged hypercall specifically to allow DOM0 to update the
    microcode on all processors (essentially just copy the Linux driver
    into Xen),
  - compile the latest microcode into Xen and have it do the update at
    boot. This would require people to upgrade Xen in order to upgrade the
    microcode.

-- 
David Hopwood <david.nospam.hopwood@blueyonder.co.uk>



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