RE: [PATCH v1 1/3] xen/arm: generalize per-page GFN storage beyond xenheap pages

["Penny, Zheng" <penny.zheng@xxxxxxx>]

[Public]

Hi,

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: Monday, March 30, 2026 9:49 PM
> To: Penny, Zheng <penny.zheng@xxxxxxx>
> Cc: Huang, Ray <Ray.Huang@xxxxxxx>; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Julien Grall <julien@xxxxxxx>; Bertrand Marquis
> <bertrand.marquis@xxxxxxx>; Orzel, Michal <Michal.Orzel@xxxxxxx>; Volodymyr
> Babchuk <Volodymyr_Babchuk@xxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> Garcia Vallejo, Alejandro <Alejandro.GarciaVallejo@xxxxxxx>
> Subject: Re: [PATCH v1 1/3] xen/arm: generalize per-page GFN storage beyond
> xenheap pages
>
> On 27.03.2026 08:50, Penny Zheng wrote:
> > As preparation for fixing mfn_to_gfn() on ARM, we extend the existing
> > GFN field in page_info's type_info to be usable for not only xenheap ones.
> > Another usage will be introduced later for stolen pages in memory exchaging.
> >
> > Introduce general-purpose page_get_gfn() and page_set_gfn() helpers
> > that read and write the GFN stored in type_info. The old
> > page_get_xenheap_gfn() and page_set_xenheap_gfn() are retained as thin
> > wrappers with their xenheap ASSERTs, so all current callers remain 
> > unchanged.
>
> Why was this GFN setting limited to Xenheap pages back at the time? Depending 
> on
> the reasons, retaining the old accessors may or may not be a good idea.
>

The only call site is to set shared_info gfn. I assumed GFN setting could be 
uniformly available.
FWIT, the only limitation is that it shall not be applied to shared pages. In 
arm, I think it's static shared memory.

> > Also introduce PGT_INVALID_GFN as the general sentinel, with
> > PGT_INVALID_XENHEAP_GFN aliased to it for backward compatibility.
>
> This I view as unnecessary, if not confusing.
>
> > --- a/xen/arch/arm/include/asm/mm.h
> > +++ b/xen/arch/arm/include/asm/mm.h
> > @@ -113,18 +113,21 @@ struct page_info
> >  #define PGT_count_mask    PG_mask(3, 3)
> >
> >  /*
> > - * Stored in bits [28:0] (arm32) or [60:0] (arm64) GFN if page is xenheap 
> > page.
> > + * Stored in bits [28:0] (arm32) or [60:0] (arm64) GFN if page is
> > + xenheap page,
> > + * or stolen ones in memory exchanging.
> >   */
>
> Does the purpose really need limiting like this? If the field covered by 
> PGT_gfn_* is
> uniformly available (see the question above), I don't see why a new 
> constraint would
> need spelling out. If it's not uniformly available, then likely the 
> description needs
> expanding as to when the new accessors are okay to use. If uniformly 
> available,
> what may want spelling out is under what conditions one can expect the field 
> to be
> properly set (until such time where it's set correctly on all guest-owned 
> pages).
>
> Jan