Re: [PATCH 13/13] xen: allow more than 512 GB of RAM for 64 bit pv-domains

[Juergen Gross <jgross@suse.com>]

On 02/18/2015 12:18 PM, David Vrabel wrote:
> On 18/02/15 06:52, Juergen Gross wrote:
>>
>> +if X86_64
>> +choice
>> +	prompt "Support pv-domains larger than 512GB"
>> +	default XEN_512GB_NONE
>> +	help
>> +	  Support paravirtualized domains with more than 512GB of RAM.
>> +
>> +	  The Xen tools and crash dump analysis tools might not support
>> +	  pv-domains with more than 512 GB of RAM. This option controls the
>> +	  default setting of the kernel to use only up to 512 GB or more.
>> +	  It is always possible to change the default via specifying the
>> +	  boot parameter "xen_512gb_limit".
>> +
>> +	config XEN_512GB_NONE
>> +		bool "neither dom0 nor domUs can be larger than 512GB"
>> +	config XEN_512GB_DOM0
>> +		bool "dom0 can be larger than 512GB, domUs not"
>> +	config XEN_512GB_DOMU
>> +		bool "domUs can be larger than 512GB, dom0 not"
>> +	config XEN_512GB_ALL
>> +		bool "dom0 and domUs can be larger than 512GB"
>> +endchoice
>> +endif
>
> This configuration option doesn't look useful to me.  Can we get rid of
> it with runtime checking.  e.g.,
>
> If dom0, enable >512G.
> If domU, enable >512G if requested by command line option /or/ toolstack
> indicates that it supports the linear p2m.

How is the toolstack supposed to indicate this?

I'd be more than happy to get rid of that option. For Dom0 you seem to
have changed your mind (you rejected enabling >512GB as default last
year).

Doing some more tests I found the command line option is problematic:
The option seems to be evaluated only after it is needed (I did the
first tests using the config option). Can we get rid of the option
even for domU? Or do I have to pre-scan the command line for the option?

> And
>
> If max_pfn < 512G, populate 3-level p2m /unless/ toolstack indicates it
> supports the linear p2m.

What about Dom0?

> People using crash analysis tools that need the 3-level p2m can clamp
> dom0 memory with the Xen command line option.  FWIW, the tool we use
> doesn't need this.

Interesting. Which tool are you using?


Juergen