Re: [PATCH v1 0/4] vfio: report NUMA nodes for device memory

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 27 Sep 2023 15:03:09 +0000
Vikram Sethi <vsethi@nvidia.com> wrote:

> > From: Alex Williamson <alex.williamson@redhat.com>
> > Sent: Wednesday, September 27, 2023 9:25 AM
> > To: Jason Gunthorpe <jgg@nvidia.com>
> > Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>; Ankit Agrawal
> > <ankita@nvidia.com>; David Hildenbrand <david@redhat.com>; Cédric Le
> > Goater <clg@redhat.com>; shannon.zhaosl@gmail.com;
> > peter.maydell@linaro.org; ani@anisinha.ca; Aniket Agashe
> > <aniketa@nvidia.com>; Neo Jia <cjia@nvidia.com>; Kirti Wankhede
> > <kwankhede@nvidia.com>; Tarun Gupta (SW-GPU) <targupta@nvidia.com>;
> > Vikram Sethi <vsethi@nvidia.com>; Andy Currid <ACurrid@nvidia.com>;
> > qemu-arm@nongnu.org; qemu-devel@nongnu.org; Gavin Shan
> > <gshan@redhat.com>; ira.weiny@intel.com; navneet.singh@intel.com
> > Subject: Re: [PATCH v1 0/4] vfio: report NUMA nodes for device memory
> > 
> > 
> > On Wed, 27 Sep 2023 10:53:36 -0300
> > Jason Gunthorpe <jgg@nvidia.com> wrote:
> >   
> > > On Wed, Sep 27, 2023 at 12:33:18PM +0100, Jonathan Cameron wrote:
> > >  
> > > > CXL accelerators / GPUs etc are a different question but who has one
> > > > of those anyway? :)  
> > >
> > > That's exactly what I mean when I say CXL will need it too. I keep
> > > describing this current Grace & Hopper as pre-CXL HW. You can easially
> > > imagine draping CXL around it. CXL doesn't solve the problem that
> > > motivates all this hackying - Linux can't dynamically create NUMA
> > > nodes.  
> > 
> > Why is that and why aren't we pushing towards a solution of removing that
> > barrier so that we don't require the machine topology to be configured to
> > support this use case and guest OS limitations?  Thanks,
> >  
> 
> Even if Linux could create NUMA nodes dynamically for coherent CXL or CXL type devices, 
> there is additional information FW knows that the kernel doesn't. For example,
> what the distance/latency between CPU and the device NUMA node is. While CXL devices
> present a CDAT table which gives latency type attributes within the device, there would still be some
> guesswork needed in the kernel as to what the end to end latency/distance is. 
> It's probably not the best outcome to just consider this generically far memory" because 
> is it further than Intersocket memory access or not matters. 
> Pre CXL devices such as for this patchset don't even have CDAT so the kernel by itself has
> no idea if this latency/distance is less than or more than inter socket memory access latency
> even.
> So specially for devices present at boot, FW knows this information and should provide it. 
> Similarly, QEMU should pass along this information to VMs for the best outcomes.  

Yeah, AFAICT we're not doing any of that in this series.  We're only
creating some number of nodes for the guest driver to make use of and
describing in the generated _DSD the set of nodes associated for use by
the device.  How many nodes are required, how the guest driver
partitions coherent memory among the nodes, and how the guest assigns a
distance to the nodes is unspecified.

A glance at the CDAT spec seems brilliant in this regard, is there such
a table for this device or could/should the vfio-pci variant driver
provide one?  I imagine this is how the VM admin or orchestration tool
would learn to configure nodes and the VMM would further virtualize
this table for the guest OS and drivers.  Thanks,

Alex