Re: Linux in VirtualPC in KVM fails

Alexander Graf
2009-01-09T19:25:43Z

On 09.01.2009, at 20:05, Anthony Liguori wrote:

> Avi Kivity wrote:
>> Kevin Wolf wrote:
>>> Hi,
>>>
>>> let's start with the scenario I tried to use: I have two levels of
>>> virtualization. On the physical hardware I run a Linux with KVM.
>>> The KVM
>>> guest is a Win2k3 VM which runs VirtualPC. In VirtualPC I try to
>>> run a
>>> Linux again (openSUSE 11.1 to be specific, but that shouldn't
>>> matter).
>>>
>>> The boot menu comes up nicely and so on, but early in the kernel
>>> boot it
>>> crashes:
>>>
>>> EIP is at kvm_deferred_mmu_op+0x46/0xbf
>>> Call Trace:
>>> [<c0117f79>] kvm_mmu_write+0x59/0x61
>>> [<c011bad9>] set_pte_vaddr+0x95/0xec
>>> [<c011b3b2>] __native_set_fixmap+0x1d/0x24
>>> [<c054ae5b>] test_wp_bit+0x24/0x6c
>>> [<c054b6b1>] mem_init+0x295/0x2b8
>>> [<c053a8a3>] start_kernel+0x262/0x31f
>>>
>>> Now obviously this is a KVM function where there should be none. The
>>> problem seems to be that VirtualPC doesn't intercept cpuid and
>>> thus the
>>> VirtualPC guest sees the KVM cpuid values where it better wouldn't.
>>> Consequently, it turns on the paravirt support for KVM which is
>>> exactly
>>> wrong and leads to the crash on the first hypercall.
>>>
>>> The guest has no chance to detect correctly if it's running
>>> directly on
>>> KVM or if there is another virtualization layer which can't emulate
>>> cpuid. So the fix must involve the mechanism itself. Alex has
>>> suggested
>>> to change the interface to use a KVM-specific MSR instead of cpuid
>>> as
>>> these should be handled by any virtualization software. I'm
>>> copying him
>>> so he can take over for the details, I just want to get the
>>> discussion
>>> started.
>>>
>>> So... Comments? Suggestions? Patches? ;-)
>>>
>>
>> Gaa. Looks like cpuid is totally broken by first-generation
>> virtualization products.
>
> Not at all. There's no reason that a JIT'ing virtualization product
> can't rewrite CPUID to a function call and then mask off unsupported
> bits. It's a bug in the virtualization product if it doesn't do this.

Isn't one of the great things about virtualization the fact that you
can do things you can on real hardware in the virtual machine? While
I'm not exactly a fan of VirtualPC, I would still like it to work in
KVM, as that's what real hardware is capable of.

Alex