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

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Message-ID: <aW-gM1wyZSL7X23y@Mac.lan>
Date: Tue, 20 Jan 2026 16:33:07 +0100
From: Roger Pau Monné <roger.pau@...rix.com>
To: xen-devel@...ts.xenproject.org
Cc: linux-kernel@...r.kernel.org, James Dingwall <james@...gwall.me.uk>,
	Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Subject: Re: [PATCH] Partial revert "x86/xen: fix balloon target
 initialization for PVH dom0"

On Tue, Jan 20, 2026 at 03:06:47PM +0100, 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.
> 

While kind of obvious, I guess this needs a:

Fixes: 87af633689ce ("x86/xen: fix balloon target initialization for PVH dom0")

So it doesn't fall through the cracks for backports.

Thanks, Roger.