Re: Linux in VirtualPC in KVM fails

Anthony Liguori
2009-01-09T19:05:29Z

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.

Regards,

Anthony Liguori