Re: [PATCH] x86/kvm: disable fast MMIO when running nested

[Paolo Bonzini <pbonzini@redhat.com>]

> > Michael and Jason, any progress on implementing a fast virtio mechanism
> > that doesn't rely on undefined behavior?
> >
> > (Encode writing instruction length into last 4 bits of MMIO address,
> >   side-channel say that accesses to the MMIO area always use certain
> >   instruction length, use hypercall, ...)
> >
> > Thanks.
> 
> No progress from my side. But we can use PIO for virtio 1.0 and it's
> faster than fast MMIO (qemu supports modern pio notification bar, we can
> make it as default). It looks to me that neither encoding nor hypercall
> will work for real hardware virtio device.

Encoding the instruction length would work, the h/w virtio devices would
just ignore it.  But... it is really ugly.

Using PIO would be a small step backwards for PCIe.  As long as the device
only needs *one* notification register (either MMIO or PIO) to initialize
successfully, it's okay.  Then if there is no PIO space you'd just fall back
to the slower MMIO notification.

Paolo