Re: pekka-vm and kvm documentation

Re: pekka-vm and kvm documentation

[Pekka Enberg]

Hi Avi,

Avi Kivity wrote:
> When you come up against something that is undocumented or badly 
> described, please complain on kvm@.  We will then update the documentation.

So one thing I'm wondering is in what mode do we enter the guest? Is 
that entirely specified with KVM_SET_REGS and KVM_SET_SREGS?

I assume we're entering real mode as per how I set up "rflags" but for 
some reason I'm seeing "cs" base being zeroed out before first 
instruction is executed with my simple kvm program which could mean that 
it's trying to use GDT (which is not set up at that point).

			Pekka

Re: pekka-vm and kvm documentation

[Avi Kivity]

On 03/25/2010 10:23 PM, Pekka Enberg wrote:
> Hi Avi,
>
> Avi Kivity wrote:
>> When you come up against something that is undocumented or badly 
>> described, please complain on kvm@.  We will then update the 
>> documentation.
>
> So one thing I'm wondering is in what mode do we enter the guest? Is 
> that entirely specified with KVM_SET_REGS and KVM_SET_SREGS?

Yes.  cr0.pe = real/protected, eflags.vm = vm86 mode, cs.d = 
prot16/prot32, efer.lme/efer.lma/cs.l = longmode.

>
> I assume we're entering real mode as per how I set up "rflags" but for 
> some reason I'm seeing "cs" base being zeroed out before first 
> instruction is executed with my simple kvm program which could mean 
> that it's trying to use GDT (which is not set up at that point).

I see you set up the cpu as if after reset (cs:ip = f000:fff0) but just 
dumped a kernel there.  That won't work, you need real code.

Try poking 'e9 fd ff' at your entry point (ffff0) and see the cpu pegged 
at 100% (1: jmp 1b) (this is probably the largest possible kernel that 
is bug-free).

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