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Re: [Xen-devel] [PATCH for-4.12 5/8] pvh/dom0: warn when dom0_mem is not set to a fixed value

On Thu, Feb 07, 2019 at 05:09:14PM +0000, George Dunlap wrote:
> On Wed, Jan 30, 2019 at 10:37 AM Roger Pau Monne <roger.pau@xxxxxxxxxx> wrote:
> >
> > There have been several reports of the dom0 builder running out of
> > memory when buildign a PVH dom0 without havingf specified a dom0_mem
> > value. Print a warning message if dom0_mem is not set to a fixed value
> > when booting in PVH mode.
> >
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > Cc: Juergen Gross <jgross@xxxxxxxx>
> > ---
> > Without this patch creating a PVH dom0 without a dom0_mem parameter
> > can result in the dom0 builder running out of memory thus leading to a
> > Xen crash. The added message gives a hit to the user about a possible
> > fix.
> But why would it run out of memory?  Wouldn't it be better to find and
> fix the root cause?

The root cause is that dom0_compute_nr_pages is not very good at
approximating the internal memory needed to build a PVH dom0, it only
takes into account the RAM regions of the p2m, but not the MMIO
regions, neither the fact that the p2m will likely contain holes.

I've had a patch series to fix this:


But I'm afraid we didn't reach consensus with Jan and at this point in
the release adding a warning message seems like the less controversial
option while I figure out how to proceed in order to fix this. Such
message will at least give a hint to users about a possible fix.

Thanks, Roger.

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