Re: A confusion about CXL in arm virt machine

[Jonathan Cameron via <qemu-devel@nongnu.org>]

On Fri, 16 Jun 2023 14:10:24 -0400
Gregory Price <gregory.price@memverge.com> wrote:

> On Fri, Jun 16, 2023 at 03:43:31PM +0800, Yuquan Wang wrote:
> > Hi, Gregory
> > 
> > There is one confusion about CXL in QEMU I hope to consult. 
> > If you have some time to look at this email, I would have better understanding of CXL 
> > emulation in QEMU.
> > 
> > On docs/system/devices/cxl.rst ,  Gregory wrote:
> > A very simple setup with just one directly attached CXL Type 3 Volatile Memory device::
> > qemu-system-aarch64 -M virt,gic-version=3,cxl=on -m 4g,maxmem=8G,slots=8 -cpu max \
> > ......
> > 
> > As the current master branch of QEMU has not yet complemented the CXL option/expansion 
> > in arm virt machine, how this example command lines worked? Or here used another branch 
> > rather than master?
> > 
> > Many thanks
> > Yuquan  
> 
> As of today, the qemu/qemu.git master branch does have the required
> patch for volatile region support: adacc814f541af9281c922e750d8ba4b90c1a73e
> 
> however, the last time i tested it on x86, the master branch was
> incapable of enabling these regions with the latest kernel (6.3.x)
> despite that kernel having sufficient support to do so.  I have not dug
> into what the discrepency between master and johnathan's working branch
> are just yet.

Events support is missing so the upstream kernel drivers won't probe
successfully.  That's queued up for merge but hasn't happened quite yet.
*fingers crossed* it should go in soon.

> 
> Last I tested cxl-2023-05-25 branch of Johnathan's fork is working on x86: 
> 
> https://gitlab.com/jic23/qemu/-/tree/cxl-2023-05-25
> 
> I have not worked with the ARM machine, but Johnathan may be able to
> comment on the state of ARM support for this code.

ARM support is not yet upstream.  There are some precursor problems we still
have to solve because arm-virt should also support device tree bindings.
See talk I gave at Linaro connect that includes some of them:
https://resources.linaro.org/en/resource/hM986DSHfoTrZ98UjpvLg1

For now, I'm carrying the arm-virt + ACPI support on the tree above.
There are a lot of things we still need to provide support for in QEMU CXL
world so for now figuring out the path forward for upstreaming ARM support
isn't at the top of my list.  I'll get back to it at somepoint - probably
next month.

Jonathan


> 
> ~Gregory