Re: KVM crash on unusual PM->RM transition

[Avi Kivity <avi@redhat.com>]

H. Peter Anvin wrote:
> Any reason to not make use of Qemu in userspace, rather than relying on
> the in-kernel interpreter for these?  The kernel interpreter is
> obviously The Right Thing to avoid frequent ping-pong into the kernel,
> but it seems to me that such a potential "long-term" situation might be
> better handled by Qemu?
>   

It's a faq; there are multiple reasons:

- it would make the kernel/user interface complicated; right now the 
model is "kvm emulates the cpu and optionally lapic, ioapic, and pit'; 
that would change to "kvm sometimes emulates the cpu, but sometimes 
doesn't".  We'd need to add a description of when userspace can jump 
back into the kernel (AMD for example is perfectly happy in real mode).

- it would require a way for userspace to access the in-kernel 
lapic/ioapic/pit, and for these components to inject interrupts into 
userspace.

- it requires anyone using kvm to implement a complete x86 emulator.

- qemu doesn't do smp.

with emulate_invalid_guest_space we're actually pretty close to running 
big real mode; but no one's working on it so it's stagnated.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.