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RE: [Xen-ia64-devel] consoles, iosapics, and device interrupts

To: "Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>
Subject: RE: [Xen-ia64-devel] consoles, iosapics, and device interrupts
From: Alex Williamson <alex.williamson@xxxxxx>
Date: Fri, 18 Nov 2005 09:52:44 -0700
Cc: xen-ia64-devel@xxxxxxxxxxxxxxxxxxx, Tristan Gingold <Tristan.Gingold@xxxxxxxx>
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On Fri, 2005-11-18 at 08:14 -0800, Magenheimer, Dan (HP Labs Fort
Collins) wrote:
> Sounds like good progress, but a couple comments:
> I don't think we should put the ACPI namespace code into Xen/ia64.
> IIRC, this is a *lot* of code and I don't think it is worth
> adding it just so we can have console input to Xen
> without going through Dom0.   I think the Cambridge Xen
> team would agree...

   I agree, I don't want to add it either.  Unfortunately ACPI is the
only way for DIG compliant boxes to describe much of the system.  It's
definitely a balance point for para-virtualization vs full
virtualization.  Ideally we could tweak ACPI namespace so that ACPI
objects are listed as not present for guests that aren't privileged to
use them.  This could include not only ACPI described UARTs, but PCI
root bridges, processors, thermal devices, buttons, etc...  This all
gets pretty ugly, but I'm not convinced we aren't going to have to go
down that path at some point.  I'll search through the main archives to
see if anyone has a plan for this.

> Is it possible to have console input routed to Xen
> through Dom0 via the virtual console?  This of course
> will be worthless before Xen boots Dom0, or if Dom0 crashes.
> But there really is limited value for direct console input
> into Xen except for debugging -- and it is a potentially
> big security hole.

   I think that's what we're trying to do.  Remove hpsim and the Linux
serial console leaving only the xen virtual console.  I suppose the
other option would be to eliminate the xen console and let dom0 talk to
the hardware directly.  This might be as simple as the hpsim fix that I
submitted, but then we lose access to getting to the debugging prompt.
The security risk of the debugging console doesn't seem so big to me.
Don't you have to have physical access to the console device (VGA or
serial)?  Adding a password to access the xen console would probably be
pragmatic, but I don't think we need to go overboard when access is
already so limited.

> Last, it might be worth promoting this discussion to
> xen-devel as there might be some useful input from
> Cambridge and the IBM Power port team.  Alex, could
> you summarize the discussion to date and post it to
> xen-devel for comment?

   I think we're currently just trying to get up to parity w/ x86.  x86
has a different architecture for interrupts, so doesn't really have the
same IOSAPIC issue.  AFAICT though, interrupts are virualized on x86.  I
need to get a better feel for the code to see what they've done around
ACPI though.  Power likely doesn't care about ACPI at all.  Thanks,


Alex Williamson                             HP Linux & Open Source Lab

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