[PATCH v4 04/14] PCI/P2PDMA: Clear ACS P2P flags for all devices behind switches

[Alex Williamson]

On Tue, 8 May 2018 21:42:27 +0000
"Stephen  Bates" <sbates at raithlin.com> wrote:

> Hi Alex
> 
> >    But it would be a much easier proposal to disable ACS when the
> > IOMMU is not enabled, ACS has no real purpose in that case.  
> 
> I guess one issue I have with this is that it disables IOMMU groups
> for all Root Ports and not just the one(s) we wish to do p2pdma on. 

But as I understand this series, we're not really targeting specific
sets of devices either.  It's more of a shotgun approach that we
disable ACS on downstream switch ports and hope that we get the right
set of devices, but with the indecisiveness that we might later
white-list select root ports to further increase the blast radius.

> >    The IOMMU and P2P are already not exclusive, we can bounce off
> > the IOMMU or make use of ATS as we've previously discussed.  We were
> >    previously talking about a build time config option that you
> > didn't expect distros to use, so I don't think intervention for the
> > user to disable the IOMMU if it's enabled by default is a serious
> > concern either.  
> 
> ATS definitely makes things more interesting for the cases where the
> EPs support it. However I don't really have a handle on how common
> ATS support is going to be in the kinds of devices we have been
> focused on (NVMe SSDs and RDMA NICs mostly). 
>
> > What you're trying to do is enabled direct peer-to-peer for
> > endpoints which do not support ATS when the IOMMU is enabled, which
> > is not something that necessarily makes sense to me.   
> 
> As above the advantage of leaving the IOMMU on is that it allows for
> both p2pdma PCI domains and IOMMU groupings PCI domains in the same
> system. It is just that these domains will be separate to each other.

That argument makes sense if we had the ability to select specific sets
of devices, but that's not the case here, right?  With the shotgun
approach, we're clearly favoring one at the expense of the other and
it's not clear why we don't simple force the needle all the way in that
direction such that the results are at least predictable.

> >  So that leaves avoiding bounce buffers as the remaining IOMMU
> > feature  
> 
> I agree with you here that the devices we will want to use for p2p
> will probably not require a bounce buffer and will support 64 bit DMA
> addressing. 
>
> > I'm still not seeing why it's terribly undesirable to require
> > devices to support ATS if they want to do direct P2P with an IOMMU
> > enabled.  
> 
> I think the one reason is for the use-case above. Allowing IOMMU
> groupings on one domain and p2pdma on another domain.... 

If IOMMU grouping implies device assignment (because nobody else uses
it to the same extent as device assignment) then the build-time option
falls to pieces, we need a single kernel that can do both.  I think we
need to get more clever about allowing the user to specify exactly at
which points in the topology they want to disable isolation.  Thanks,

Alex