Re: [Xen-devel] [PATCH 05 of 23] libxl: drop 8M slack for PV guests

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2012-02-21 at 10:53 +0000, David Vrabel wrote:
> On 21/02/12 10:10, Ian Campbell wrote:
> > On Tue, 2012-02-21 at 10:10 +0000, Stefano Stabellini wrote:
> >> On Mon, 20 Feb 2012, Ian Jackson wrote:
> >>> Ian Campbell writes ("[Xen-devel] [PATCH 05 of 23] libxl: drop 8M slack 
> >>> for PV guests"):
> >>>> libxl: drop 8M slack for PV guests.
> >>>>
> >>>> As far as I can tell this serves no purpose. I think it relates to the 
> >>>> old
> >>>> 8M to "account for backend allocations" which we used to add. This 
> >>>> leaves a bit
> >>>> of unpopulated space in the Pseudo-physical address space which can be 
> >>>> used by
> >>>> backends when mapping foreign memory. However 8M is not representative 
> >>>> of that
> >>>> any more and modern kernels do not operate in this way anyway.
> >>>>
> >>>> I suspect an argument could be made for removing this from the libxl API
> >>>> altogether but instead lets just set the overhead to 0.
> >>>
> >>> I think this is plausible but I'd like to hear from Stefano, who iirc
> >>> may have some knowledge about the reason for this 8Mb.
> >>
> >> It comes from XenD (and XAPI too, if I recall correctly), see this
> >> comment in tools/python/xen/xend/image.py:
> > 
> > That doesn't answer the "why" though.
> > 
> > I think I should just be more assertive since I believe I do actually
> > know why the 8 is there (or certainly nobody appears to know any
> > better). I'll update the changelog to read:
> > 
> >         This serves no purpose. It relates to the old 8M to "account for
> >         backend allocations" which we used to add. This leaves a bit of
> >         unpopulated space in the Pseudo-physical address space which can
> >         be used by backends when mapping foreign memory. However 8M is
> >         not representative of that any more and modern kernels do not
> >         operate in this way anyway.
> 
> Should a similar fix be applied to Xen as well for dom0?

I don't think it is harmful to have the extra 8, but yes we could
consider doing so.

Ian.


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