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

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:

class X86_Linux_ImageHandler(LinuxImageHandler):

    def buildDomain(self):
        # set physical mapping limit
        # add an 8MB slack to balance backend allocations.

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