Re: KVM crash on unusual PM->RM transition

Avi Kivity
2009-04-14T16:06:51Z

> 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.