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

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
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 ...


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