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

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Message-ID: <1357dda0-bb5d-4f59-a53c-d584099bf65c@amd.com>
Date: Thu, 22 Jan 2026 12:03:47 -0500
From: Jason Andryuk <jason.andryuk@....com>
To: Roger Pau Monné <roger.pau@...rix.com>
CC: <xen-devel@...ts.xenproject.org>, <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 2026-01-22 11:57, Roger Pau Monné wrote:
> On Thu, Jan 22, 2026 at 09:40:01AM -0500, Jason Andryuk wrote:
>> On 2026-01-21 12:49, Roger Pau Monné wrote:
>>> I haven't tested it yet to see whether that's OK to do on PV, I would
>>> think PV and PVH would be the same here, since the setting of the
>>> xenstore target value is based in the return of
>>> XENMEM_current_reservation for both.
>>
>> On a system with 32GB and dom0=pvh dom0_mem=7G:
>>
>> [    0.295201] xen:balloon: current_pages: 1835007 get_num_physpages 8220126
>> xen_released_pages 6385120
>> [    0.295201] ------------[ cut here ]------------
>> [    0.295201] Released pages underflow current target
>>
>> 8220126 - 6385120 = 1835006
>>
>> And also for PV:
>>
>> [    1.406923] xen:balloon: current_pages: 1835008 get_num_physpages 8220127
>> xen_released_pages 6385120
>> [    1.406928] ------------[ cut here ]------------
>> [    1.406931] Released pages underflow current target
>>
>>
>> So we don't want to subtract xen_released_pages for dom0.  Is
>> xen_released_pages expected to be non-zero for a domU?
> 
> Oh, yes.  In fact I think the patch here is wrong for PV dom0, as it
> shouldn't subtract xen_released_pages from xen_start_info->nr_pages.
> I will need to send v2.

To be clear, the numbers and warning are from the follow on 
current_reservation patch.

Regards,
Jason