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

[Richard Cheng]

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.

Best regards,
Richard Cheng.
 
> > 
> > Signed-off-by: Richard Cheng <icheng@xxxxxxxxxx>
> > Acked-by: Kai-Heng Feng <kaihengf@xxxxxxxxxx>
> > ---
> >  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;
>