Xen users - Aug 2007 - Xen and Intel Debug Registers

Hi All,

Has anyone had issues with using CPU Debug Registers inside a VM
running under Xen?

I am getting inconsistent results when attempting to use the Intel
Debug Registers (DR0-DR7) in a Debian VM running inside a Xen 3.0.3
install.  I have found that after setting a breakpoint address
register (DR0-DR3) and correctly setting the debug control register
(DR7) two problems sometimes occur:

1. The breakpoint address register is randomly reset to 0
2. The control register is randomly reset to 0 (or 0x00000400)

This does not always happen, but often enough to be annoying.

If I run the exact same code on a domain 0 (non-virtualized) debian
install, the breakpoint and control register values function as
desired, every time.

I would love to hear if anyone else had had success or failure when
attempting to use Debug Registers inside a Xen VM.

Thanks,
Matt

p.s.  I''m running my Xen install on a dual-core intel cpu

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On Thu, 16 Aug 2007 16:51:20 -0400
"Matt Evenson" <mevenson@komoku.com> wrote:
> I would love to hear if anyone else had had success or failure when
> attempting to use Debug Registers inside a Xen VM.
Yes indeed, there appear to be problems. At work we have a big
opteron smp system we have lots of different linux distributions
on in HVMs to use for testing our products in multiple environments.
When I run many debugger tests in parallel on different HVMs,
I get random failures in the watchpoint tests (watchpoints
use the debug registers to generate address traps). If I rerun
the failed tests on one HVM at a time with the other idle, they
always work fine. (And then there are the rhel 5 HVMs which almost
always crash during the debugger tests - don''t know if it is the
same problem or not).

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