Re: [Xen-devel] [PATCH v3 4/8] xen: Use typesafe gfn in xenmem_add_to_physmap_one

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 23/06/16 11:20, Stefano Stabellini wrote:
> On Tue, 21 Jun 2016, Julien Grall wrote:
> > The x86 version of the function xenmem_add_to_physmap_one contains
> > variable name gpfn and gfn which make the code very confusing.
> > I have left unchanged for now.
> >
> > Also, rename gpfn to gfn in the ARM version as the latter is the correct
> > acronym for a guest physical frame.
> >
> > Finally, remove the trailing whitespace around the changes.
> >
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> > Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> >
> > ---
> > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> > Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > Cc: Tim Deegan <tim@xxxxxxx>
> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> >
> >      Changes in v3:
> >          - Add Jan's acked-by for non-ARM bits
> > ---
> >   xen/arch/arm/mm.c    | 10 +++++-----
> >   xen/arch/x86/mm.c    | 15 +++++++--------
> >   xen/common/memory.c  |  6 +++---
> >   xen/include/xen/mm.h |  2 +-
> >   4 files changed, 16 insertions(+), 17 deletions(-)
> >
> > diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> > index 5ab9b75..6882d54 100644
> > --- a/xen/arch/arm/mm.c
> > +++ b/xen/arch/arm/mm.c
> > @@ -1046,7 +1046,7 @@ int xenmem_add_to_physmap_one(
> >       unsigned int space,
> >       union xen_add_to_physmap_batch_extra extra,
> >       unsigned long idx,
> > -    xen_pfn_t gpfn)
> > +    gfn_t gfn)
> I think there is a possible loss of information here: xen_pfn_t is
> always uint64_t on both ARM and ARM64, while gfn_t is unsigned long with
> its current definition. Or am I missing something?
>
> In fact, given that ARM supports LPAE, shouldn't gfn by defined as
> xen_pfn_t in terms of storage size (TYPE_SAFE(xen_pfn_t, gfn)) ?
With LPAE, ARM32 supports up to 40-bit PA so the frame will be encoded 
on 28-bit. So unsigned long is perfectly fine here.
Note that the truncation already occurs in subsequent caller because we 
are using unsigned long for the P2M parameters.
> >   {
> >       unsigned long mfn = 0;
> >       int rc;
> > @@ -1081,8 +1081,8 @@ int xenmem_add_to_physmap_one(
> >               else
> >                   return -EINVAL;
> >           }
> > -
> > -        d->arch.grant_table_gpfn[idx] = gpfn;
> > +
> > +        d->arch.grant_table_gpfn[idx] = gfn_x(gfn);
> Similarly grant_table_gpfn is an array of xen_pfn_t (uint64_t), while
> gfn_x unboxes to the storage size of gfn (unsigned long).
Patch #5 will switch grant_table_gpfn to an array of gfn_t.

Regards,

--
Julien Grall

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