Re: [Xen-devel] [PATCH] xl.cfg: document the maxmem= option

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Ian Campbell [mailto:Ian.Campbell@xxxxxxxxxx]
> Sent: Tuesday, May 08, 2012 6:41 AM
> To: Dan Magenheimer
> Cc: Olaf Hering; xen-devel@xxxxxxxxxxxxxxxxxxx; Konrad Wilk
> Subject: RE: [Xen-devel] [PATCH] xl.cfg: document the maxmem= option
> 
> On Mon, 2012-05-07 at 17:37 +0100, Dan Magenheimer wrote:
> > > From: Ian Campbell [mailto:Ian.Campbell@xxxxxxxxxx]
> > > Sent: Sunday, May 06, 2012 3:36 AM
> > > To: Olaf Hering
> > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Konrad Rzeszutek Wilk
> > > Subject: Re: [Xen-devel] [PATCH] xl.cfg: document the maxmem= option
> > >
> > > On Fri, 2012-05-04 at 16:18 -0400, Olaf Hering wrote:
> > > > On Fri, May 04, Konrad Rzeszutek Wilk wrote:
> > > >
> > > > > On Fri, May 04, 2012 at 09:31:42PM +0200, Olaf Hering wrote:
> > > > > > # HG changeset patch
> > > > > > # User Olaf Hering <olaf@xxxxxxxxx>
> > > > > > # Date 1336159720 -7200
> > > > > > # Node ID c414728d0d12f1c3e416e40cceefca2f0b00578e
> > > > > > # Parent  8f556a70ae0bef47e242f9e7be0a054769fc8277
> > > > > > xl.cfg: document the maxmem= option
> > > > > >
> > > > > > Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
> > > > > >
> > > > > > diff -r 8f556a70ae0b -r c414728d0d12 docs/man/xl.cfg.pod.5
> > > > > > --- a/docs/man/xl.cfg.pod.5
> > > > > > +++ b/docs/man/xl.cfg.pod.5
> > > > > > @@ -484,6 +484,14 @@ are not using hardware assisted paging (
> > > > > >  mode) and your guest workload consists of a a very large number of
> > > > > >  similar processes then increasing this value may improve 
> > > > > > performance.
> > > > > >
> > > > > > +=item B<maxmem=MBYTES>
> > > > > > +
> > > > > > +Specifies the maximum amount of memory the HVM guest can ever see.
> >
> > can _ever_ see...
> >
> > What about guest OS's that support hotplug memory?
> 
> Does the toolstack (need to) expose the capability to drive that as a
> host admin? I don't think xl does... In which case this wording can be
> tweaked when that feature is added?

I don't know if the matching support ever was implemented in xl,
but hotplug memory support for Xen is in upstream Linux.

http://lists.xen.org/archives/html/xen-devel/2011-05/msg01060.html 

If it _is_ in xl, "can ever see" seems a bit too strong... maybe
something like "memory the HVM guest can see unless changed later
via 'xl mem-max'" or "memory the HVM guest can see when it boots"
would be better.

But this is a nit.  It's impossible to both be precise and brief
in descriptions such as these.

> > Also, a subtle point, but is this the maximum physical address that
> > will contain read/write pages or the maximum amount of RAM that the
> > guest OS should be prepared to map as read/writeable (e.g. after
> > subtracting off reserved e820 ranges) or ???
> 
> I think it is total number of memory pages, not the maximum PFN. e.g.
> the maximum address might be 4.5GB on a 4GB machine with a PCI hole.
> 
> I *think*...

OK.  If so, then the proposed wording is fine, though maybe adding
one word "maximum amount of _usable_ memory" might help.

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