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

[Jason Andryuk <jason.andryuk@xxxxxxx>]

On 2026-01-21 06:17, Roger Pau Monné wrote:
> 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.
Yes, I thought that could be a follow on.  I've attached what I have 
tested, but it is based on a branch pre-dating xen_released_pages. 
xenmem_current_reservation with PVH dom0 seemed good without the 
xen_released_pages adjustment, but I don't know what that would be for a 
PVH dom0.
Regards,
Jason

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