Re: [Xen-devel] [PATCH v9] xen/grant-table: Avoid m2p_override during mapping

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 20 Feb 2014, David Vrabel wrote:
> On 20/02/14 18:17, Stefano Stabellini wrote:
> > On Thu, 20 Feb 2014, Zoltan Kiss wrote:
> >> On 20/02/14 17:26, Stefano Stabellini wrote:
> >>> On Mon, 17 Feb 2014, Zoltan Kiss wrote:
> >>>> On 16/02/14 18:36, Stefano Stabellini wrote:
> >>>>> On Wed, 12 Feb 2014, Zoltan Kiss wrote:
> >>>>>> diff --git a/arch/arm/include/asm/xen/page.h
> >>>>>> b/arch/arm/include/asm/xen/page.h
> >>>>>> index e0965ab..4eaeb3f 100644
> >>>>>> --- a/arch/arm/include/asm/xen/page.h
> >>>>>> +++ b/arch/arm/include/asm/xen/page.h
> >>>>>> @@ -97,16 +97,15 @@ static inline pte_t *lookup_address(unsigned long
> >>>>>> address, unsigned int *level)
> >>>>>>        return NULL;
> >>>>>>    }
> >>>>>>
> >>>>>> -static inline int m2p_add_override(unsigned long mfn, struct page
> >>>>>> *page,
> >>>>>> -              struct gnttab_map_grant_ref *kmap_op)
> >>>>>> -{
> >>>>>> -      return 0;
> >>>>>> -}
> >>>>>> -
> >>>>>> -static inline int m2p_remove_override(struct page *page, bool
> >>>>>> clear_pte)
> >>>>>> -{
> >>>>>> -      return 0;
> >>>>>> -}
> >>>>>> +extern int set_foreign_p2m_mapping(struct gnttab_map_grant_ref
> >>>>>> *map_ops,
> >>>>>> +                                 struct gnttab_map_grant_ref
> >>>>>> *kmap_ops,
> >>>>>> +                                 struct page **pages, unsigned int
> >>>>>> count,
> >>>>>> +                                 bool m2p_override);
> >>>>>> +
> >>>>>> +extern int clear_foreign_p2m_mapping(struct gnttab_unmap_grant_ref
> >>>>>> *unmap_ops,
> >>>>>> +                                   struct gnttab_map_grant_ref
> >>>>>> *kmap_ops,
> >>>>>> +                                   struct page **pages, unsigned int
> >>>>>> count,
> >>>>>> +                                   bool m2p_override);
> >>>>>
> >>>>> Much much better.
> >>>>> The only comment I have is about this m2p_override boolean parameter.
> >>>>> m2p_override is now meaningless in this context, what we really want to
> >>>>> let the arch specific implementation know is whether the mapping is a
> >>>>> kernel only mapping or a userspace mapping.
> >>>>> Testing for kmap_ops != NULL might even be enough, but it would not
> >>>>> improve the interface.
> >>>> gntdev is the only user of this, the kmap_ops parameter there is:
> >>>> use_ptemod ? map->kmap_ops + offset : NULL
> >>>> where:
> >>>> use_ptemod = !xen_feature(XENFEAT_auto_translated_physmap);
> >>>> So I think we can't rely on kmap_ops to decide whether we should
> >>>> m2p_override
> >>>> or not.
> >>>>
> >>>>> Is it possible to realize if the mapping is a userspace mapping by
> >>>>> checking for GNTMAP_application_map in map_ops?
> >>>>> Otherwise I would keep the boolean and rename it to user_mapping.
> >>>> Sounds better, but as far as I see gntdev set that flag in
> >>>> find_grant_ptes,
> >>>> which is called only
> >>>>
> >>>> if (use_ptemod) {
> >>>>  err = apply_to_page_range(vma->vm_mm, vma->vm_start,
> >>>>                            vma->vm_end - vma->vm_start,
> >>>>                            find_grant_ptes, map);
> >>>>
> >>>> So if xen_feature(XENFEAT_auto_translated_physmap), we don't have
> >>>> kmap_ops,
> >>>> and GNTMAP_application_map is not set as well, but I guess we still need
> >>>> m2p_override. Or not? I'm a bit confused, maybe because of Monday ...
> >>>
> >>> If xen_feature(XENFEAT_auto_translated_physmap) we shouldn't need the
> >>> m2p_override.
> >>>
> >>
> >> So it's safe to assume that we need m2p_override only if kmap_ops != NULL, 
> >> and
> >> we can avoid the extra bool parameter, is that correct? At least with the
> >> current users of grant mapping it seems to be true.
> >> In which case we don't need the wrappers for gnttab_[un]map_refs as well.
> >> Actually the most of m2p_add/remove_override takes effect only if there is 
> >> a
> >> kmap_op parameter, but what about the rest of the code there?
> > 
> > It is safe to assume that we only need the m2p_override if
> > !xen_feature(XENFEAT_auto_translated_physmap).
> > I wouldn't make any assumptions on kmap_ops != NULL.
> 
> I think it is -- we only need the m2p override if we have userspace
> mappings (where kmap_ops != 0).
>
> > I would remove the bool m2p_override parameter completely and determine
> > whether we need to call the m2p_override functions from the x86
> > implementation of set/clear_foreign_p2m_mapping by checking
> > xen_feature(XENFEAT_auto_translated_physmap).
> > 
> > David, does it seem reasonable to you?
> 
> That would miss the point of this patch which is to avoid adding to the
> m2p_override for kernel only mappings.

I meant checking 

!xen_feature(XENFEAT_auto_translated_physmap) && kmap_ops != 0

At least this way the "hack" would be entirely self contained in the
arch specific code.

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