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Re: [Xen-devel] [PATCH v1] xen/arm: Do not allocate pte entries for MAP_SMALL_PAGES

Hi Ian,

On 24/02/2015 09:31, Ian Campbell wrote:
On Wed, 2015-02-18 at 13:03 +0000, Julien Grall wrote:
+                {
+                    pte = mfn_to_xen_entry(mfn, (ai & 0xffff));

Please introduce a new macro for the mask.

Better would be a pte_foo accessor, similar (if not identical) to x86's
pte_get_flags. So pte_get_flags(ai) or so.

I'm not able to find a such function in x86. Did you intend to mean pte_flags_to_cacheattr?

+                    pte.pt.table = 1;
+                    write_pte(&third[third_table_offset(addr)], pte);
+                }
               case REMOVE:
                   if ( !third[third_table_offset(addr)].pt.valid )
diff --git a/xen/include/asm-arm/page.h b/xen/include/asm-arm/page.h
index 3e7b0ae..80415b3 100644
--- a/xen/include/asm-arm/page.h
+++ b/xen/include/asm-arm/page.h
@@ -61,10 +61,16 @@
   #define DEV_WC        BUFFERABLE

+/* bit 16 in the Attribute index can be used to know if
+ * PTE entry should be added or not. This is useful
+ * when ONLY non-leaf page table entries need to allocated
+ */
+#define PTE_INVALID   (0x1 << 16)

It makes more sense to introduce a PTE_PRESENT flags compare to
PTE_INVALID. The former has more meaning that the latter.

Agreed that PTE_PRESENT is the way to go. Ideally this could all be done
via the lpae_pt_t type, but I suspect that might turn out to be tricky.

I don't think it's a good idea to re-use lpae_pt_t type for this purpose. We might decide to introduce flags which won't be set in the final PTE.

In another side, using PTE_PRESENT would require to introduce a PAGE_AVAIL0 (or smth similar).

#define PTE_PRESENT ((struct lpae_t){ .pt.present = 1 }).bits

probably doesn't work, I'm not even sure if this sort of thing is
possible. If not then "#define PTE_PRESET (1ULL<<0)".

The attribute index (write-alloc, buferrable...) is using the less significant 3 bits. So I was suggesting to use the top of the word.


Julien Grall

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