Re: [PATCH] kvm: x86: disable KVM_FAST_MMIO_BUS

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 16, 2017 at 07:19:28PM +0200, Paolo Bonzini wrote:
> On 16/08/2017 18:50, Michael S. Tsirkin wrote:
> > On Wed, Aug 16, 2017 at 03:30:31PM +0200, Paolo Bonzini wrote:
> >> While you can filter out instruction fetches, that's not enough.  A data
> >> read could happen because someone pointed the IDT to MMIO area, and who
> >> knows what the VM-exit instruction length points to in that case.
> > 
> > Thinking more about it, I don't really see how anything
> > legal guest might be doing with virtio would trigger anything
> > but a fault after decoding the instruction. How does
> > skipping instruction even make sense in the example you give?
> 
> There's no such thing as a legal guest.  Anything that the hypervisor
> does, that differs from real hardware, is a possible escalation path.

Fast MMIO bus devices don't apprear out of thin air.

They appear because guest enabled a virtio device.

So it is a PV guest and if it doesn't behave according to the virtio
spec, it is going to crash.


> 
> This in fact makes me doubt the EMULTYPE_SKIP patch too.
> 
> >>>> Plus of course it wouldn't be guaranteed to work on nested.
> >>>
> >>> Not sure I got this one.
> >>
> >> Not all nested hypervisors are setting the VM-exit instruction length
> >> field on EPT violations, since it's documented not to be set.
> > 
> > So that's probably the real issue - nested virt which has to do it
> > in software at extra cost. We already limit this to intel processors,
> > how about we blacklist nested virt for this optimization?
> > 
> > I agree it's skating it a bit close to the dangerous edge,
> > but so are other tricks we play with PTEs to speed up MMIO.
> 
> Not at all.  Everything else we do is perfectly fine according to the
> spec, this one isn't.
> 
> Paolo

Virtio MMIO is kind of special in many ways.

What happens if I map and try to execute an MMIO BAR? I don't think it
will work, will it?


> >>>>>> Adding a hypercall or MSR write that does a fast MMIO write to a physical
> >>>>>> address would do it, but it adds hypervisor knowledge in virtio, including
> >>>>>> CPUID handling.
> >>>>>
> >>>>> Another issue is that it will break DPDK on virtio.
> >>>>
> >>>> Not break, just make it slower.
> >>>
> >>> I thought hypercalls can only be triggered from ring 0, userspace can't call them.
> >>> Dod I get it wrong?
> >>
> >> That's just a limitation that KVM makes on currently-defined hypercalls.
> >>
> >> VMCALL causes a vmexit if executed from ring 3.
> >>
> >> Paolo
> >