Re: [RFC PATCH kernel] vfio-pci: Allow write combining

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Mon, 2017-10-16 at 22:11 +1100, Alexey Kardashevskiy wrote:
> On 16/10/17 19:38, Benjamin Herrenschmidt wrote:
> > On Mon, 2017-10-16 at 18:36 +1100, Alexey Kardashevskiy wrote:
> > > 
> > > 1. Allow msix mapping to the userspace (to address non-64k-aligned msix bar)
> > > 
> > > 2. Allow write combining in vfio for the userspace (kvm guest is kinda
> > > special and may simply ignore mapping flags in some configs but PPC radix
> > > guests still rely on this)
> > 
> > Why ? The "G" bit is entirely under control of the guest afaik.
> 
> Yes, for hash guests. I am not sure sure about radix, Paul pointed me to
> the code in KVM which uses the VFIO's mapping VMA.

With radix, the HW will honor the G bit set in the guest page tables.

> 
> > This
> > would only affect qemu itself. It's still useful for things like dpdk
> > using vfio.
> 
> Correct, this is useful regardless KVM.
> 
> > > 3. what callback and where needs to be added to inform HV/PR KVM about VFIO
> > > group, like IOMMUMR::add_vfio_group() proposal or something.
> > 
> > Can you elaborate a bit ? I haven't followed this.
> 
> David knows :)
> 
> 
>