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

Michael S. Tsirkin
2018-01-25T17:11:12Z

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.

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MST