Re: [PATCH] Partial revert "x86/xen: fix balloon target initialization for PVH dom0"

[Roger Pau Monné]

On Tue, Jan 20, 2026 at 03:10:06PM -0500, Jason Andryuk wrote:
> On 2026-01-20 09:06, Roger Pau Monne wrote:
> > This partially reverts commit 87af633689ce16ddb166c80f32b120e50b1295de so
> > the current memory target for PV guests is still fetched from
> > start_info->nr_pages, which matches exactly what the toolstack sets the
> > initial memory target to.
> > 
> > Using get_num_physpages() is possible on PV also, but needs adjusting to
> > take into account the ISA hole and the PFN at 0 not considered usable
> > memory depite being populated, and hence would need extra adjustments.
> > Instead of carrying those extra adjustments switch back to the previous
> > code.  That leaves Linux with a difference in how current memory target is
> > obtained for HVM vs PV, but that's better than adding extra logic just for
> > PV.
> > 
> > Also, for HVM the target is not (and has never been) accurately calculated,
> > as in that case part of what starts as guest memory is reused by hvmloader
> > and possibly other firmware to store ACPI tables and similar firmware
> > information, thus the memory is no longer being reported as RAM in the
> > memory map.
> > 
> > Reported-by: James Dingwall <james@xxxxxxxxxxxxxx>
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>

Thanks.

I've been considering what we discussed and as a separate follow up we
might want to attempt to switch to using `XENMEM_current_reservation`
for dom0?  It might make the accounting in PVH dom0 better, as that's
what the toolstack uses to set the xenstore target when initializing
dom0 values.

Regards, Roger.