Re: [PATCH] tools/testing/cxl: Align mock CFMWS to PMD_SIZE for ARM64 64K pages

[Richard Cheng <icheng@nvidia.com>]

On Wed, May 20, 2026 at 10:26:04PM +0800, Alison Schofield wrote:
> On Thu, May 21, 2026 at 11:09:09AM +0800, Richard Cheng wrote:
> > On Wed, May 20, 2026 at 11:18:23AM +0800, Dave Jiang wrote:
> > > 
> > > 
> > > On 5/19/26 4:35 PM, Richard Cheng wrote:
> > > > cxl_test allocate synthetic CFMWS HPA windows out of a gen_pool and asks
> > > > the allocator for SZ_256M alignment. It has been sufficient on x86 with
> > > > 4k pages, arm64 with 4k pages since their PMD_SIZE are 2MB.
> > > > 
> > > > But for 64k-page arm64 kernel with CONFIG_ARM64_64K_PAGES=y and
> > > > CONFIG_PGTABLE_LEVELS=3 , the PMD_SIZE is 512 MB, which is much larger
> > > > than the alignment cxl_test guarantees. That results in every CXL region
> > > > carved from that window inherits a mis-aligned start.
> > > > 
> > > > The DAX driver's cxl_dax_region_probe() then calls "alloc_dax_region()
> > > > and the probe fails with -ENOMEM, with error message
> > > > 
> > > > """
> > > >     cxl_dax_region dax_region1: probe with driver cxl_dax_region failed
> > > >                                 with error -12
> > > > """
> > > > 
> > > > It was hit while bringing up cxl_test on an ARM64 server with
> > > > ARM64_64K_PAGES config.
> > > > 
> > > > Raise the alignment passed to "alloc"mock_res()" to the larger of
> > > > SZ_256M and PMD_SIZE so that the mock CFMWS window is always at least as
> > > > well-aligned as "alloc_dax_region()" require.
> > > 
> > > https://sashiko.dev/#/patchset/20260519233523.5991-1-icheng%40nvidia.com
> > > 
> > > Do you need to adjust length as well as alloc_dax_region() also checks length alignment?
> > > 
> > > DJ
> > >
> > 
> > Hi Dave,
> > 
> > Thanks for the review. I think there're 2 things I would like to mention
> > * cfmws6's window_size is SZ_256M * 8UL, which is 2G, not 256M, only cfmws5 is 256M, so the
> > population in your scenario is narrower.
> > * The -ENOMEM comes from cxl_dax_region_probe() on the auto region, which carves mock_auto_region_size out of cfmws0,
> > which is 1G. On ARM64K_PAGES + PGTABLE_LEVELS=3, range_len == SZ_512M == PMD_SIZE and range.start == cfmws0.base_hpa.
> > Both length and start checks in alloc_dax_region() pass.
> > 
> > A region carved explicitly from cfmws5 with a non-PMD-aligned length would still be rejected, but that path was
> > already borken on this config and isn't exercised by cxl_test bringup.
> > I'll be happy to send a follow-up path that bumps cfmws5 to >=PMD_SIZE , if you think that case should be covered, too.
> > Though that's separate from the bug this patch addresses.
> > 
> 
> Consider not limiting the scope of this fix to only what
> is needed to load cxl-test module for arm64. Think of that
> issue as a motivating example to create a new rule.
> 
> Would something like this work for you?
>

Hi Alison,

Thanks for the feedback, it sounds good for me.
 
> cxl/test: Enforce PMD alignment for volatile mock regions
> 
> Problem description stays as it is now.
> 
> Plan to resolve is a new rule like...
> 
> Every volatile mock CFMWS must be PMD aligned in start and size so 
> DAX on RAM works on any supported page size config. The auto region
> carved from cfmws0 takes size and offset from mock_auto_region_size,
> so that value must be PMD aligned too.
> 

Agree.

> And to enforce that rule do 3 things:
> 1. cfmws5's window_size becomes max(SZ_256M, PMD_SIZE)
> 2. populate_cedt() aligns volatile windows to max(SZ_256M, PMD_SIZE)
> 3. cxl_test_init() check mock_auto_region_size alignment
> 
> To be clear, this suggestion is to limit applying the alignment
> to cfmws's w ACPI_CEDT_CFMWS_RESTRICT_VOLATILE. PMEM windows stay
> at SZ_256M because IIUC they don't need PMD alignment.
> 
> -- Alison
> 

No problem, I'll amend these things and make the fix more generic
in version 2.

Best regards,
Richard Cheng.


> > Best regards,
> > Richard Cheng.
> >  
> > > > 
> > > > Signed-off-by: Richard Cheng <icheng@nvidia.com>
> > > > Acked-by: Kai-Heng Feng <kaihengf@nvidia.com>
> > > > ---
> > > >  tools/testing/cxl/test/cxl.c | 3 ++-
> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/tools/testing/cxl/test/cxl.c b/tools/testing/cxl/test/cxl.c
> > > > index 418669927fb0..b40e4bbcc958 100644
> > > > --- a/tools/testing/cxl/test/cxl.c
> > > > +++ b/tools/testing/cxl/test/cxl.c
> > > > @@ -497,7 +497,8 @@ static int populate_cedt(void)
> > > >  		struct acpi_cedt_cfmws *window = mock_cfmws[i];
> > > >  
> > > >  		cfmws_elc_update(window, i);
> > > > -		res = alloc_mock_res(window->window_size, SZ_256M);
> > > > +		res = alloc_mock_res(window->window_size,
> > > > +				     max_t(int, SZ_256M, PMD_SIZE));
> > > >  		if (!res)
> > > >  			return -ENOMEM;
> > > >  		window->base_hpa = res->range.start;
> > > 
> >