Re: [Xen-devel] [PATCH v3] xen/grant-table: Avoid m2p_override during mapping

On 21/01/14 12:26, Stefano Stabellini wrote:

On Mon, 20 Jan 2014, Zoltan Kiss wrote:

quoted

-		ret = m2p_add_override(mfn, pages[i], kmap_ops ?
-				       &kmap_ops[i] : NULL);
+		if (m2p_override)
+			ret = m2p_add_override(mfn, pages[i], kmap_ops ?
+					       &kmap_ops[i] : NULL);
+		else {
+			unsigned long pfn = page_to_pfn(pages[i]);
+			WARN_ON(PagePrivate(pages[i]));
+			SetPagePrivate(pages[i]);
+			set_page_private(pages[i], mfn);
+			pages[i]->index = pfn_to_mfn(pfn);
+			if (unlikely(!set_phys_to_machine(pfn, FOREIGN_FRAME(mfn))))
+				return -ENOMEM;

What happens if the page is PageHighMem?

This looks like a subset of m2p_add_override, but it is missing some
relevant bits, like the PageHighMem check, or the p2m(m2p(mfn)) == mfn
check.  Maybe we can find a way to avoid duplicating the code.
We could split m2p_add_override in two functions or add yet another
parameter to it.

The PageHighMem() check isn't relevant as we're not mapping anything
here.  Also, a page for a kernel grant mapping only cannot be highmem.

The check for a local mfn and the additional set_phys_to_machine() is
only necessary if something tries an mfn_to_pfn() on the local mfn.  We
can only omit adding an m2p override if we know thing will try
mfn_to_pfn(), therefore the check and set_phys_to_machine() is unnecessary.

David