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

[Jason Wang]

On 2018å01æ26æ 01:11, Michael S. Tsirkin wrote:
> On Thu, Jan 25, 2018 at 09:49:22AM -0500, Paolo Bonzini wrote:
> > > > 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
> A bigger issue for PIO is it's causing exits for hw devices.

Just to make sure I understand. For exits you mean vmexit? I believe 
MMIO will cause vmexit too.

Thanks