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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Jan 21, 2026 at 12:21:33PM -0500, Jason Andryuk wrote:
> 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@dingwall.me.uk>
> > > > Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> > > 
> > > Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
> > 
> > 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

> From 8b628ad0ebe52c30e31298e868f2f5187f2f52da Mon Sep 17 00:00:00 2001
> From: Jason Andryuk <jason.andryuk@amd.com>
> Date: Fri, 7 Nov 2025 16:44:53 -0500
> Subject: [PATCH] xen/balloon: Initialize dom0 with XENMEM_current_reservation
> 
> The balloon driver bases its action off the memory/target and
> memory/static-max xenstore keys.  These are set by the toolstack and
> match the domain's hypervisor allocated pages - domain_tot_pages().
> 
> However, PVH and HVM domains query get_num_physpages() for the initial
> balloon driver current and target pages.  get_num_physpages() is different
> from dom_totain_pages(), and has been observed way off in a PVH dom0.
> Here a PVH dom0 is assigned 917503 pages (3.5GB), but
> get_num_physpages() is 7312455:
> xen:balloon: pages curr 7312455 target 7312455
> xen:balloon: current_reservation 917503
> 
> While Xen truncated the PVH dom0 memory map's RAM, Linux undoes that
> operation and restores RAM regions in pvh_reserve_extra_memory().
> 
> Use XENMEM_current_reveration to initialize the balloon driver current
> and target pages.  This is the hypervisor value, and matches what the
> toolstack writes.  This avoids any ballooning from the inital
> allocation.  While domUs are affected, the implications of the change
> are unclear - only make the change for dom0.
> 
> Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
> ---
>  drivers/xen/balloon.c          | 9 ++++++---
>  drivers/xen/mem-reservation.c  | 8 ++++++++
>  include/xen/interface/memory.h | 5 +++++
>  include/xen/mem-reservation.h  | 2 ++
>  4 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index 528395133b4f..fa6cbe6ce2ca 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -713,10 +713,13 @@ static int __init balloon_init(void)
>  
>  #ifdef CONFIG_XEN_PV
>  	balloon_stats.current_pages = xen_pv_domain()
> -		? min(xen_start_info->nr_pages - xen_released_pages, max_pfn)
> -		: get_num_physpages();
> +		? min(xen_start_info->nr_pages - xen_released_pages, max_pfn) :
> +		xen_initial_domain() ? xenmem_current_reservation() :
> +				       get_num_physpages();
>  #else
> -	balloon_stats.current_pages = get_num_physpages();
> +	balloon_stats.current_pages =
> +		xen_initial_domain() ? xenmem_current_reservation() :
> +				       get_num_physpages();
>  #endif
>  	balloon_stats.target_pages  = balloon_stats.current_pages;
>  	balloon_stats.balloon_low   = 0;
> diff --git a/drivers/xen/mem-reservation.c b/drivers/xen/mem-reservation.c
> index 24648836e0d4..40c5c40d34fe 100644
> --- a/drivers/xen/mem-reservation.c
> +++ b/drivers/xen/mem-reservation.c
> @@ -113,3 +113,11 @@ int xenmem_reservation_decrease(int count, xen_pfn_t *frames)
>  	return HYPERVISOR_memory_op(XENMEM_decrease_reservation, &reservation);
>  }
>  EXPORT_SYMBOL_GPL(xenmem_reservation_decrease);
> +
> +long xenmem_current_reservation(void)
> +{
> +	struct xen_memory_domain domain = { .domid = DOMID_SELF };
> +
> +	return HYPERVISOR_memory_op(XENMEM_current_reservation, &domain);
> +}
> +EXPORT_SYMBOL_GPL(xenmem_current_reservation);
> diff --git a/include/xen/interface/memory.h b/include/xen/interface/memory.h
> index 1a371a825c55..72619a75fed2 100644
> --- a/include/xen/interface/memory.h
> +++ b/include/xen/interface/memory.h
> @@ -104,6 +104,11 @@ DEFINE_GUEST_HANDLE_STRUCT(xen_memory_exchange);
>   */
>  #define XENMEM_maximum_ram_page     2
>  
> +struct xen_memory_domain {
> +    /* [IN] Domain information is being queried for. */
> +    domid_t domid;
> +};

Other callers that would use xen_memory_domain just pass a pointer to
a domid_t, I think you could simplify the patch to the diff below,
which sits on top of the patch here:

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.

Thanks, Roger.
---
diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index e799650f6c8c..c592d7bae8c3 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -724,7 +724,8 @@ static int __init balloon_add_regions(void)
 static int __init balloon_init(void)
 {
 	struct task_struct *task;
-	unsigned long current_pages;
+	unsigned long current_pages = 0;
+	domid_t domid = DOMID_SELF;
 	int rc;
 
 	if (!xen_domain())
@@ -732,8 +733,13 @@ static int __init balloon_init(void)
 
 	pr_info("Initialising balloon driver\n");
 
-	current_pages = xen_pv_domain() ? min(xen_start_info->nr_pages, max_pfn)
-	                                : get_num_physpages();
+	if (xen_initial_domain())
+		current_pages = HYPERVISOR_memory_op(XENMEM_current_reservation,
+		                                     &domid);
+	if (current_pages <= 0)
+		current_pages =
+		    xen_pv_domain() ? min(xen_start_info->nr_pages, max_pfn)
+	                            : get_num_physpages();
 
 	if (xen_released_pages >= current_pages) {
 		WARN(1, "Released pages underflow current target");