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

[George Dunlap <dunlapg@xxxxxxxxx>]

On Thu, Feb 7, 2019 at 5:48 PM Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>
> 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:
>
> https://lists.xenproject.org/archives/html/xen-devel/2018-12/msg00437.html
>
> 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.

OK; then both the changelog and the code (via a comment) should
clearly indicate that this is a temporary work-around; and ideally the
log message too.

What about something like the following? "WARNING: PVH dom0 without
dom0_mem set is still unstable.  If you get crashes during boot, try
adding a dom0_mem parameter."

Thanks,
 -George

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