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

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Nov 06, 2017 at 04:44:03PM +1100, Alexey Kardashevskiy wrote:
> On 16/10/17 19:01, David Gibson wrote:
> > On Mon, Oct 16, 2017 at 06:36:29PM +1100, Alexey Kardashevskiy wrote:
> >> On 16/10/17 17:00, David Gibson wrote:
> >>> On Mon, Oct 16, 2017 at 04:54:08PM +1100, Alexey Kardashevskiy wrote:
> >>>> On 12/10/17 02:35, Benjamin Herrenschmidt wrote:
> >>>>> On Wed, 2017-10-11 at 13:56 +1100, Alexey Kardashevskiy wrote:
> >>>>>> Oopsie. This is because I overlooked it. Others do not use it. So I do need
> >>>>>> a file. But in the current scheme where all BARs share one fd - it won't
> >>>>>> work - I simply cannot allow WC on non-prefetchable BARs :-/
> >>>>>
> >>>>> This is an oversight in the design of VFIO-PCI, it should have a way to
> >>>>> specify write combine, either implicitely via such an arch hook, or
> >>>>> explicitely via an ioctl prior to mapping the BARs for example.
> >>>>>
> >>>>> Alex, what do you reckon is the best approach here ?
> >>>>
> >>>> /me wonders if it is yet another issue for the dead issues bucket, just
> >>>> like the msix mapping one :)
> >>>
> >>> Maybe.  Alexey, maybe you can make up a list of things that we (me,
> >>> you, BenH) need to discuss with Alex W at KVM Forum?
> >>
> >> "you" - you meant me? I am not coming over there :(
> > 
> > Oh.. I thought you were.
> > 
> >> The list is:
> >>
> >> 1. Allow msix mapping to the userspace (to address non-64k-aligned msix bar)

We have a new plan on this - I'll discuss it over IRC.

> >> 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)

AIUI this isn't for radix, but for DPDK things that we need this.  Ben
talked about it a bit, but I don't know what the outcome was.

> >> 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.

This was discussed, and I'm still thinking about it.  It's kind of
curly.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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