Re: Plan 9 install problems

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

James Morris wrote:
> I just tried installing Plan 9 from the current CD ISO (see 
> http://plan9.bell-labs.com/plan9/download.html) under KVM and didn't get 
> very far.
>
> It boots, then hangs after some initialization.  Screenshot of the guest 
> at this point: http://namei.org/images/p9-kvm.png
>
> On the host, the startup messages look like:
>
> set_vram_mapping: memory: e0000000 - e0800000
> set_vram_mapping: return 0x2b03ced87000
> vga_update_vram: base 0x2b03ced87000 ptr 0x2b03ce585010
> vga_update_vram: done
>
> kernel: kvm: unhandled rdmsr: 1
>
>   

This is an old Penium msr related to machine checks.

> When the guest hangs, the host starts logging this repeatedly: 
>
> kernel: double fault 0x80031b30 @ 0xf01003c9
>   

Since we don't handle the rdmsr, we inject a general protection fault.  
The guest isn't ready to handle this, so we inject a double fault.  The 
guest isn't ready to handle a double fault either -- at this point we 
should triple fault but we don't.

> This is with the 2.6.20-rc1 kernel and kvm-6 userland.
>
> Seems to work as expected with the kvm kernel module unloaded (i.e. plain 
> qemu).
>   

The way to fix is to add handling to {svm,vmx}_get_msr() for msr 1 (and 
any other pointless msrs the guest reads or writes).  From a cursory 
reading of the machine check docs, returning 0 should be fine for msr 1.

[those common msrs should really be moved to kvm_main.c one day]

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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