Re: [PATCH v4 3/3] acpi,srat: give memory block size advice based on CFMWS alignment

[Gregory Price]

On Wed, Oct 30, 2024 at 11:40:08AM +0100, David Hildenbrand wrote:
> On 29.10.24 21:20, Gregory Price wrote:
> > Capacity is stranded when CFMWS regions are not aligned to block size.
> > On x86, block size increases with capacity (2G blocks @ 64G capacity).
> > 
> > Use CFMWS base/size to report memory block size alignment advice.
> > 
> > Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
> > ---
> >   drivers/acpi/numa/srat.c | 19 +++++++++++++++++--
> >   1 file changed, 17 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/acpi/numa/srat.c b/drivers/acpi/numa/srat.c
> > index 44f91f2c6c5d..a24aff38c465 100644
> > --- a/drivers/acpi/numa/srat.c
> > +++ b/drivers/acpi/numa/srat.c
> > @@ -14,6 +14,7 @@
> >   #include <linux/errno.h>
> >   #include <linux/acpi.h>
> >   #include <linux/memblock.h>
> > +#include <linux/memory.h>
> >   #include <linux/numa.h>
> >   #include <linux/nodemask.h>
> >   #include <linux/topology.h>
> > @@ -338,12 +339,26 @@ static int __init acpi_parse_cfmws(union acpi_subtable_headers *header,
> >   {
> >   	struct acpi_cedt_cfmws *cfmws;
> >   	int *fake_pxm = arg;
> > -	u64 start, end;
> > +	u64 start, end, align, size;
> >   	int node;
> >   	cfmws = (struct acpi_cedt_cfmws *)header;
> >   	start = cfmws->base_hpa;
> > -	end = cfmws->base_hpa + cfmws->window_size;
> > +	size = cfmws->window_size;
> > +	end = cfmws->base_hpa + size;
> > +
> > +	/* Align memblock size to CFMW regions if possible */
> > +	for (align = SZ_64T; align >= SZ_256M; align >>= 1) {
> > +		if (IS_ALIGNED(start, align) && IS_ALIGNED(size, align))
> > +			break;
> > +	}
> 
> Are there maybe some nice tricks bi-tricks to avoid the loop and these
> hardcoded limits? :)
> 
> align = 1UL << __ffs(start | end));
> 
> Assuming "unsigned long" is sufficient in this code (64bit) and "start |
> end" will never be 0.
>

This will work, if start | end is < 256MB, the ACPI table is invalid by
definition since either the block itself is <256MB or the size is 0 (which
is nonsense).  So yeah i can simplify here. 

Ack. will push v5 once i get KLP to clear another warning.
 
> -- 
> Cheers,
> 
> David / dhildenb
>