Re: [PATCH V5 1/4] ACPI: Support Generic Initiator only domains

[Dan Williams]

On Wed, Nov 13, 2019 at 5:57 AM Tao Xu <tao3.xu@xxxxxxxxx> wrote:
>
> On 11/13/2019 5:47 PM, Jonathan Cameron wrote:
> > On Tue, 12 Nov 2019 09:55:17 -0800
> > Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >
> >> [ add Tao Xu ]
> >>
> >> On Fri, Oct 4, 2019 at 4:45 AM Jonathan Cameron
> >> <Jonathan.Cameron@xxxxxxxxxx> wrote:
> >>>
> >>> Generic Initiators are a new ACPI concept that allows for the
> >>> description of proximity domains that contain a device which
> >>> performs memory access (such as a network card) but neither
> >>> host CPU nor Memory.
> >>>
> >>> This patch has the parsing code and provides the infrastructure
> >>> for an architecture to associate these new domains with their
> >>> nearest memory processing node.
> >>
> >> Thanks for this Jonathan. May I ask how this was tested? Tao has been
> >> working on qemu support for HMAT [1]. I have not checked if it already
> >> supports generic initiator entries, but it would be helpful to include
> >> an example of how the kernel sees these configurations in practice.
> >>
> >> [1]: http://patchwork.ozlabs.org/cover/1096737/
> >
> > Tested against qemu with SRAT and SLIT table overrides from an
> > initrd to actually create the node and give it distances
> > (those all turn up correctly in the normal places).  DSDT override
> > used to move an emulated network card into the GI numa node.  That
> > currently requires the PCI patch referred to in the cover letter.
> > On arm64 tested both on qemu and real hardware (overrides on tables
> > even for real hardware as I can't persuade our BIOS team to implement
> > Generic Initiators until an OS is actually using them.)
> >
> > Main real requirement is memory allocations then occur from one of
> > the nodes at the minimal distance when you are do a devm_ allocation
> > from a device assigned. Also need to be able to query the distances
> > to allow load balancing etc.  All that works as expected.
> >
> > It only has a fairly tangential connection to HMAT in that HMAT
> > can provide information on GI nodes.  Given HMAT code is quite happy
> > with memoryless nodes anyway it should work.  QEMU doesn't currently
> > have support to create GI SRAT entries let alone HMAT using them.
> >
> > Whilst I could look at adding such support to QEMU, it's not
> > exactly high priority to emulate something we can test easily
> > by overriding the tables before the kernel reads them.
> >
> > I'll look at how hard it is to build an HMAT tables for my test
> > configs based on the ones I used to test your HMAT patches a while
> > back.  Should be easy if tedious.
> >
> > Jonathan
> >
> Indeed, HMAT can support Generic Initiator, but as far as I know, QEMU
> only can emulate a node with cpu and memory, or memory-only. Even if we
> assign a node with cpu only, qemu will raise error. Considering
> compatibility, there are lots of work to do for QEMU if we change NUMA
> or SRAT table.

Thanks for the background. It would still be a useful feature to be
able to define a memory + generic-initiator node in qemu. That will
mirror real world accelerators with local memory configurations.