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

[Juergen Gross <jgross@xxxxxxxx>]

On 02/19/2015 06:51 PM, David Vrabel wrote:
> On 18/02/2015 11:51, Juergen Gross wrote:
> > 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 don't know.  A bit in the shared info perhaps?
Okay, I'd be fine with this. CC'ed the hypervisor maintainers to enable
them to object early. :-)

> > 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).
> Since for dom0 it's only crash analysis that might be impacted, removing
> the option is fine.  Anyone using such a tool probably know what they're
> doing.
Fine, I'll remove it.

> > 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?
> I don't what people to be able to inadvertantly create domUs that cannot
> be saved/migrated.  Can you think of another mechanism?
No, I hoped you'd have an idea.

I'll try to parse the command line options a little bit earlier.

> > > And
> > >
> > > If max_pfn < 512G, populate 3-level p2m /unless/ toolstack indicates it
> > > supports the linear p2m.
> > What about Dom0?
> This bit is for dom0 and domU.
Okay, but which toolstack would set the indicator for dom0? The
hypervisor? Would it set this bit in any case, or depending on a
command line option?


Juergen

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