Re: CXL emulation on aarch64

[Jonathan Cameron]

On Fri, 17 Jan 2025 09:43:11 +0000
Jonathan Cameron via <qemu-devel@nongnu.org> wrote:

> On Fri, 17 Jan 2025 10:13:41 +0900
> Itaru Kitayama <itaru.kitayama@linux.dev> wrote:
> 
> > > On Jan 16, 2025, at 19:58, Jonathan Cameron <Jonathan.Cameron@huawei.com> 
> > > wrote:
> > > 
> > > On Thu, 16 Jan 2025 15:04:53 +0900
> > > Itaru Kitayama <itaru.kitayama@linux.dev> wrote:
> > >     
> > >> Hi Jonathan,
> > >>     
> > >>> On Jan 14, 2025, at 19:26, Jonathan Cameron 
> > >>> <Jonathan.Cameron@huawei.com> wrote:
> > >>> 
> > >>> On Tue, 14 Jan 2025 12:03:03 +0900
> > >>> Itaru Kitayama <itaru.kitayama@linux.dev> wrote:
> > >>>     
> > >>>> Hi Jonathan, 
> > >>>>     
> > >>>>> On Jan 10, 2025, at 21:31, Jonathan Cameron 
> > >>>>> <Jonathan.Cameron@huawei.com> wrote:
> > >>>>> 
> > >>>>> On Fri, 10 Jan 2025 09:20:54 +0000
> > >>>>> "Zhijian Li (Fujitsu)" via <qemu-devel@nongnu.org> wrote:
> > >>>>>     
> > >>>>>> On 10/01/2025 13:29, Itaru Kitayama wrote:        
> > >>>>>>> Hi,
> > >>>>>>> Is anybody working on the CXL emulation on aarch64?          
> > >>>>>> 
> > >>>>>> I'm not currently working on the CXL emulation on aarch64.
> > >>>>>> 
> > >>>>>> However, IIRC the CXL maintainer's tree should work.
> > >>>>>> https://gitlab.com/jic23/qemu/        
> > >>>>> 
> > >>>>> Pick up latest branch from there. I'm prepping a rebased version
> > >>>>> with some new stuff but might take a few more days.        
> > >>>> 
> > >>>> Thanks for sharing your work with us.  Your master and cxl-2024-11-27 
> > >>>> branches give:
> > >>>> 
> > >>>> $ qemu-system-aarch64: -accel tcg,cxl=on: Property 'tcg-accel.cxl' not 
> > >>>> found      
> > >>> 
> > >>> cxl is a machine property not a accel one. So needs to be after virt
> > >>> There are tests in the tree for bios tables. Copy the command line from 
> > >>> those.
> > >>>     
> > >>>> 
> > >>>> My commands are below:
> > >>>> $HOME/projects/qemu/build/qemu-system-aarch64 \
> > >>>>       -M virt,virtualization=on,gic-version=3 \
> > >>>>       -M acpi=off -cpu max,sme=off -m 8G -smp 4 \
> > >>>>       -accel tcg,cxl=on \
> > >>>>       -nographic \
> > >>>>       -bios $HOME/cca-v4/out/bin/flash.bin \
> > >>>>       -kernel Image-cca \
> > >>>>       -drive 
> > >>>> format=raw,if=none,file=$HOME/cca-v4/out-or/images/rootfs.ext2,id=hd0 \
> > >>>>       -device virtio-blk-pci,drive=hd0 \
> > >>>>       -append root=/dev/vda \
> > >>>>       -nodefaults \
> > >>>>       --serial tcp:localhost:54320 \
> > >>>>        -serial tcp:localhost:54321 \
> > >>>>        -append "root=/dev/vda earlycon console=hvc0" \
> > >>>>        -device virtio-net-pci,netdev=net0 \
> > >>>>        -netdev user,id=net0 \
> > >>>>        -device virtio-9p-device,fsdev=shr0,mount_tag=shr0 \
> > >>>>        -fsdev local,security_model=none,path=../../,id=shr0
> > >>>> 
> > >>>> Yes, I’m using Linaro’s CCA capable OP-TEE builds above.      
> > >>> 
> > >>> I'm a little curious why optee is relevant for this but shouldn't 
> > >>> matter as long
> > >>> as an appropriate EDK2 is loaded.
> > >>>     
> > >> 
> > >> I picked up your tree’s “master” and “cxl-next” as of today, and only 
> > >> the latter at least booted.
> > >> The former gives:
> > >> 
> > >> qemu-system-aarch64: Property 'virt-9.2-machine.cxl' not found
> > >> 
> > >> Should I stick with the cxl-next? My concern is that the base QEMU 
> > >> version is a bit old
> > >> 7.0.50.    
> > > 
> > > Always use the latest dated branch on that tree.  I release whenever there
> > > is something new to carry or a major rebase needed.
> > > 
> > > cxl-<date> is the right branch to use. Hope that helps.    
> > 
> > When do you think you want to get them (aarch64 specific?) merged mainline. 
> > Any reason you want to carry the patches by yourself?  
> 
> Nothing much has changed since I presented on this at Linaro connect in 2023.
> https://resources.linaro.org/en/resource/hM986DSHfoTrZ98UjpvLg1
> 
> The issue is device tree bindings for PCI Expander bridgess and the fact that
> those need to be generated without the full enumeration that EDK2 is doing
> prior to ACPI final table builds. In order to move forward with that it
> needs a bunch of work to prove that we absolutely cannot get patches
> upstream to support kernel base enumeration and breaking up of the
> various resources (like EDK2 does).

I was talking to Peter Maydell earlier and given developments in the last couple
of years that have by necessity been ACPI only in arm virt he is less
opposed to ACPI only features being added where device tree is challenging.

So we may be able to move forwards without device tree support.

The PXB enumeration question is also relevant for managing multiple
vIOMMUs to represent multiple physical IOMMUs with the correct isolation
and do it efficiently which is probably a more pressing usecase than CXL 
emulation.
The discussion was mainly about that usecase, but maybe it also unblocks
upstreaming this support.

Thanks,

Jonathan

> 
> Given PXB enumeration in kernel has some issues on ARM anyway (that you can 
> paper
> over with _DSM 5 - it self requiring an extra patch that isn't upstreamable 
> because
> of IO port issues) there is quite a bit of work needed, mostly not in QEMU.
> Or convince Peter and others that not all virt support needs DT bindings
> (note that PXB for PCIE has been supported for years without an DT support,
> just no one noticed!)
> 
> After that we'd need to figure out CXL DT bindings in general and add kernel
> code support - despite there being no known DT based CXL systems out there, so
> that is going to be hard to do.  Various CXL kernel maintainers have expressed
> they aren't against such support, but it's hardly going to be review priority
> (other than for me if someone else does the work!)
> 
> For me this isn't particularly high priority. The ARM bit is fairly easy to 
> rebase.
> I would like to see it solved, but it is behind various other items on my
> backlog.
> 
> There are SBSA machine patches on list, but it's not a useful platform for
> CXL kernel code development because of the limited supported configurations
> (in keeping with the more or less fixed model that SBSA-ref uses).
> 
> Jonathan
> 
> 
> 
> > 
> > Itaru.
> >   
> > > 
> > > Jonathan
> > >     
> > >> 
> > >> Thanks,
> > >> Itaru.
> > >>     
> > >>> Jonathan
> > >>>     
> > >>>> 
> > >>>> Let me know which branch you were suggesting.
> > >>>> 
> > >>>> Thanks,
> > >>>> Itaru. 
> > >>>>     
> > >>>>> 
> > >>>>> Note my main development work is on arm64 so that tends to work
> > >>>>> more reliably than x86 which I only lightly test for stuff that
> > >>>>> isn't ready for upstream yet.
> > >>>>> 
> > >>>>> Give me a shout if you run into any problems.
> > >>>>> 
> > >>>>> The main blocker on upstreaming this is resolving the missing device 
> > >>>>> tree
> > >>>>> support for PCI expander bridges.  I've not made any progress on this 
> > >>>>> since
> > >>>>> talk at Linaro connect in 2023.
> > >>>>> 
> > >>>>> Jonathan
> > >>>>> 
> > >>>>>     
> > >>>>>> 
> > >>>>>> 
> > >>>>>> Thanks
> > >>>>>> Zhijian
> > >>>>>>     
> > >>>>>>> If there’s a WIP branch, a pointer would be appreciated.
> > >>>>>>> 
> > >>>>>>> Itaru          
> > 
> >   
> 
>