Re: State of debug register emulation

Joerg Roedel
2008-04-29T14:40:54Z

On Tue, Apr 29, 2008 at 03:07:25PM +0200, Jan Kiszka wrote:
> Hi,
>
> looks like we are getting better and better here in hitting yet
> unsupported corner-case features of KVM :). This time our guest fiddles
> with hardware debugging registers, but quickly gets unhappy as they do
> not yet have the expected effect.

KVM is mostly tested with guests that run with paging. So a 16 bit
protected mode guest is not tested very well :)

> Joerg, I found you SVM-related patch series in the archive which does
> not seem to have raised much responses. Is this general direction OK?
> Does it allow self-debugging of guests? But how are conflicts resolved
> if both guest and host need the physical registers (host debugging the
> guest which is debugging itself)?

I sent a patchset in the past to enable guest debugging for SVM which
means debugging the guest from outside using gdb. But I was not able to
test these patches because the userspace side of guest debugging is
broken in the kvm-qemu.
Debugging in the guest should work without problems. The debug registers
are switched between guest and host if the guest uses them. So there
should be no problems when the guest and the host using the debug
registers.

> I would try to dig into the VMX side if the general architecture is
> -mostly- clear. [ Sorry, Joerg, someone put the latter type of HW on my
> desk :->. Hope I can once check our stuff against SVM as well! ]

With some debug output from SVM I can better help to debug your
problems ;-)

Joerg

--
| AMD Saxony Limited Liability Company & Co. KG
Operating | Wilschdorfer Landstr. 101, 01109 Dresden, Germany
System | Register Court Dresden: HRA 4896
Research | General Partner authorized to represent:
Center | AMD Saxony LLC (Wilmington, Delaware, US)
| General Manager of AMD Saxony LLC: Dr. Hans-R. Deppe, Thomas McCoy