Subject: Re: [RFC FIX PATCH v0] powerpc,numa: Fix memory_hotplug_max()

From: Nathan Fontenot <[email-masked]>
Date: Fri, 8 Apr 2016 00:27:44 -0500

On 04/06/2016 04:44 AM, Bharata B Rao wrote:
> memory_hotplug_max() uses hot_add_drconf_memory_max() to get maxmimum
> addressable memory by referring to ibm,dyanamic-memory property. There
> are three problems with the current approach:
> 
> 1 hot_add_drconf_memory_max() assumes that ibm,dynamic-memory includes
>   all the LMBs of the guest, but that is not true for PowerKVM which
>   populates only DR LMBs (LMBs that can be hotplugged/removed) in that
>   property.
> 2 hot_add_drconf_memory_max() multiplies lmb-size with lmb-count to arrive
>   at the max possible address. Since ibm,dynamic-memory doesn't include
>   RMA LMBs, the address thus obtained will be less than the actual max
>   address. For example, if max possible memory size is 32G, with lmb-size
>   of 256MB there can be 127 LMBs in ibm,dynamic-memory (1 LMB for RMA
>   which won't be present here).  hot_add_drconf_memory_max() would then
>   return the max addressable memory as 127 * 256MB = 31.75GB, the max
>   address should have been 32G which is what ibm,lrdr-capacity shows.
> 3 In PowerKVM, there can be a gap between the end of boot time RAM and
>   beginning of hotplug RAM area. So just multiplying lmb-count with
>   lmb-size will not provide the correct max possible address for PowerKVM.
> 
> This patch fixes 1 by using ibm,lrdr-capacity property to return the max
> addressable memory whenever the property is present. Then it fixes 2 & 3
> by fetching the address of the last LMB in ibm,dynamic-memory property.
> 
> NOTE: There are some unnecessary changes in the patch because of converting
> spaces to tabs w/o which checkpatch.pl complains.
> 
> Signed-off-by: Bharata B Rao <[email-masked]>
> ---
>  arch/powerpc/mm/numa.c | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
> index 669a15e..57d5877 100644
> --- a/arch/powerpc/mm/numa.c
> +++ b/arch/powerpc/mm/numa.c
> @@ -1164,17 +1164,32 @@ int hot_add_scn_to_nid(unsigned long scn_addr)
>  static u64 hot_add_drconf_memory_max(void)
>  {
>          struct device_node *memory = NULL;
> -        unsigned int drconf_cell_cnt = 0;
> -        u64 lmb_size = 0;
> +	struct device_node *dn = NULL;
> +	unsigned int drconf_cell_cnt = 0;
> +	u64 lmb_size = 0;
>  	const __be32 *dm = NULL;
> +	const __be64 *lrdr = NULL;
> +	struct of_drconf_cell drmem;
> +
> +	dn = of_find_node_by_path("/rtas");
> +	if (dn) {
> +		lrdr = of_get_property(dn, "ibm,lrdr-capacity", NULL);
> +		of_node_put(dn);
> +		if (lrdr)
> +			return be64_to_cpup(lrdr);
> +	}
>  
>          memory = of_find_node_by_path("/ibm,dynamic-reconfiguration-memory");
>          if (memory) {
> -                drconf_cell_cnt = of_get_drconf_memory(memory, &dm);
> -                lmb_size = of_get_lmb_size(memory);
> -                of_node_put(memory);
> -        }
> -        return lmb_size * drconf_cell_cnt;
> +		drconf_cell_cnt = of_get_drconf_memory(memory, &dm);
> +		lmb_size = of_get_lmb_size(memory);
> +
> +		/* Advance to the last cell, each cell has 6 32 bit integers */
> +		dm += (drconf_cell_cnt - 1) * 6;

You could do this as follows to avoid hard-coding 6
		dm += (drconf_cell_cnt - 1) * sizeof(struct of_drconf_cell)

> +		read_drconf_cell(&drmem, &dm);
> +		of_node_put(memory);
> +	}
> +	return drmem.base_addr + lmb_size;

I assume it is a safe assumption that there will only be 1 RMA LMB?

I do see that the PAPR defines a bit in the flags field for each LMB
in ibm,dynamic-memory as 'reserved'. Is this something you could use
to flag RMA LMBs and put them in the ibm,dynamic-memory property?

I'm just curious why these LMBs are not in this property.

-Nathan 
>  }
>  /*