Re: [PATCH v2] x86/xen: fix balloon target initialization for PVH dom0

[Jürgen Groß <jgross@xxxxxxxx>]

On 04.04.25 15:34, Roger Pau Monne wrote:
> PVH dom0 re-uses logic from PV dom0, in which RAM ranges not assigned to
> dom0 are re-used as scratch memory to map foreign and grant pages.  Such
> logic relies on reporting those unpopulated ranges as RAM to Linux, and
> mark them as reserved.  This way Linux creates the underlying page
> structures required for metadata management.
>
> Such approach works fine on PV because the initial balloon target is
> calculated using specific Xen data, that doesn't take into account the
> memory type changes described above.  However on HVM and PVH the initial
> balloon target is calculated using get_num_physpages(), and that function
> does take into account the unpopulated RAM regions used as scratch space
> for remote domain mappings.
>
> This leads to PVH dom0 having an incorrect initial balloon target, which
> causes malfunction (excessive memory freeing) of the balloon driver if the
> dom0 memory target is later adjusted from the toolstack.
>
> Fix this by using xen_released_pages to account for any pages that are part
> of the memory map, but are already unpopulated when the balloon driver is
> initialized.  This accounts for any regions used for scratch remote
> mappings.
>
> Take the opportunity to unify PV with PVH/HVM guests regarding the usage of
> get_num_physpages(), as that avoids having to add different logic for PV vs
> PVH in both balloon_add_regions() and arch_xen_unpopulated_init().
>
> Much like a6aa4eb994ee, the code in this changeset should have been part of
> 38620fc4e893.
>
> Fixes: a6aa4eb994ee ('xen/x86: add extra pages to unpopulated-alloc if 
> available')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>


Juergen

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