Re: [PATCH] docs/cxl: Add serial number for persistent-memdev

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

On Fri, 14 Mar 2025 08:44:05 -0700
Dan Williams <dan.j.williams@intel.com> wrote:

> Jonathan Cameron wrote:
> > On Wed, 5 Mar 2025 18:35:40 +0800
> > Yuquan Wang <wangyuquan1236@phytium.com.cn> wrote:
> >   
> > > > 
> > > > On Tue, 4 Mar 2025 14:22:48 +0800
> > > > Yuquan Wang <wangyuquan1236@phytium.com.cn> wrote:
> > > >     
> > > > > > 
> > > > > > On Thu, Feb 20, 2025 at 04:12:13PM +0000, Jonathan Cameron wrote:      
> > > > > > > On Mon, 17 Feb 2025 19:20:39 +0800
> > > > > > > Yuquan Wang <wangyuquan1236@phytium.com.cn> wrote:
> > > > > > >       
> > > > > > > > Add serial number parameter in the cxl persistent examples.
> > > > > > > > 
> > > > > > > > Signed-off-by: Yuquan Wang <wangyuquan1236@phytium.com.cn>      
> > > > > > > Looks good.  I've queued it up on my gitlab staging tree, but
> > > > > > > Michael if you want to pick this one directly that's fine as well.      
> > > > > > 
> > > > > > See no reason to, I was not even CC'd.      
> > > > > 
> > > > > Hi, Michael
> > > > > 
> > > > > I'm sorry, this is my fault. I used "get_maintainer.pl" to check this
> > > > > patch's maintainers but it shows "No maintainers found, printing recent
> > > > > contributors". 
> > > > >     
> > > > I usually stage up multiple series together and send on to Michael.
> > > > So it was be being lazy for a minor change rather than anything much
> > > > that you did wrong.
> > > > 
> > > > If I get time I'll post a series with this a few other patches
> > > > later today.  
> > > > 
> > > > Jonathan
> > > >     
> > > Thank you!
> > > 
> > > BTW, I found a corner case in CXL numa node creation.
> > > 
> > > Condition: 
> > > 1) A UMA/NUMA system without SRAT, but with CEDT.CFMWS
> > > 2）Enable CONFIG_ACPI_NUMA
> > > 
> > > Results:
> > > 1) acpi_numa_init: the fake_pxm will be 0 and send to acpi_parse_cfmws()
> > > 2）If dynamically create cxl ram region, the cxl memory would be assigned
> > > to node0 rather than a new node
> > > 
> > > Confusions:
> > > 1) Is a numa system a requirement for CXL memory usage?  
> > 
> > Obviously discussion has gone on elsewhere, but I'd say in general it
> > would be a bad idea to not have an SRAT because the moment we add CXL
> > it is definitely a NUMA system and we want the Generic Port entry to
> > allow us to get perf information.
> > 
> > So I wouldn't mind if we fail CXL init in this case, but maybe
> > it is worth papering over things.  
> 
> I think that is too severe. If a driver has a path to advertise
> resources, even in a less than ideal way, it should make every effort to
> do that. There are plenty of ways for the NUMA information to fail, that
> does not mean the memory needs to be prevented from coming online. Let
> the end user decide if lack of performance information is fatal.

You are too nice to those firmware folk ;)  How will they learn!

Everything in default node is fine.  I don't much like the having
broken normal numa setup combined with CXL trying to carry on with
its node. I suspect that will be fragile in the long run.

numa_off and other things set in that path tend to spread there wings
into surprising places and broken / missing SRAT + CXL is something
that isn't likely to get much testing.

Jonathan


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