Re: [Xen-devel] [PATCH v12 05/11] x86/mm: add HYPERVISOR_memory_op to acquire guest resources

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Paul,

On 27/10/17 16:19, Paul Durrant wrote:
> > -----Original Message-----
> > From: Julien Grall [mailto:julien.grall@xxxxxxxxxx]
> > Sent: 27 October 2017 12:46
> > To: Jan Beulich <JBeulich@xxxxxxxx>; Paul Durrant
> > <Paul.Durrant@xxxxxxxxxx>
> > Cc: Julien Grall <julien.grall@xxxxxxx>; Andrew Cooper
> > <Andrew.Cooper3@xxxxxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>; George
> > Dunlap <George.Dunlap@xxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>;
> > Stefano Stabellini <sstabellini@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> > Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Daniel De Graaf
> > <dgdegra@xxxxxxxxxxxxx>; Tim (Xen.org) <tim@xxxxxxx>
> > Subject: Re: [Xen-devel] [PATCH v12 05/11] x86/mm: add
> > HYPERVISOR_memory_op to acquire guest resources
> >
> > Hi,
> >
> > On 26/10/17 16:39, Jan Beulich wrote:
> > > > > > On 26.10.17 at 17:32, <julien.grall@xxxxxxxxxx> wrote:
> > > > On 26/10/17 16:26, Jan Beulich wrote:
> > > > > > > > On 17.10.17 at 15:24, <paul.durrant@xxxxxxxxxx> wrote:
> > > > > > +    /* IN/OUT - If the tools domain is PV then, upon return, frame_list
> > > > > > +     *          will be populated with the MFNs of the resource.
> > > > > > +     *          If the tools domain is HVM then it is expected that, on
> > > > > > +     *          entry, frame_list will be populated with a list of GFNs
> > > > > > +     *          that will be mapped to the MFNs of the resource.
> > > > > > +     *          If -EIO is returned then the frame_list has only been
> > > > > > +     *          partially mapped and it is up to the caller to unmap all
> > > > > > +     *          the GFNs.
> > > > > > +     *          This parameter may be NULL if nr_frames is 0.
> > > > > > +     */
> > > > > > +    XEN_GUEST_HANDLE(xen_ulong_t) frame_list;
> > > > > This is still xen_ulong_t, which I can live with, but then you shouldn't
> > > > > copy into / out of arrays of other types in acquire_resource() (the
> > > > > more that this is common code, and iirc xen_ulong_t and
> > > > > unsigned long aren't the same thing on ARM32).
> > > > xen_ulong_t is always 64-bit on Arm (32-bit and 64-bit). But shouldn't
> > > > we use xen_pfn_t here?
> > > I had put this question up earlier, but iirc Paul didn't like it.
> > I'd like to understand why Paul doesn't like it. We should never assume
> > that a frame fit in xen_ulong_t. xen_pfn_t was exactly introduced for
> > that purpose.
> My reservation is whether xen_pfn_t is intended to hold either gfns or mfns, 
> since this hypercall uses the same array for both. If it suitable then I am 
> happy to change it, but Andrew led me to believe otherwise.
Looking at the public hearders, xen_pfn_t is been used for both MFN (see 
xenpf_add_memtype) and GFN (see gnttab_setup_table).
So I think it would be fine to do the same here.

Cheers,

--
Julien Grall

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