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SubjectRe: [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:
>
>> - 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


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