[Xen-devel] [PATCH] xen: mark local pages as FOREIGN in the m2p_override

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

When the frontend and the backend reside on the same domain, even if we
add pages to the m2p_override, these pages will never be returned by
mfn_to_pfn because the check "get_phys_to_machine(pfn) != mfn" will
always fail, so the pfn of the frontend will be returned instead
(resulting in a deadlock because the frontend pages are already locked).

However m2p_add_override can easily find out whether another pfn
corresponding to the mfn exists in the m2p, and can set the FOREIGN bit
in the p2m, making sure that mfn_to_pfn returns the pfn of the backend.

This allows the backend to perform direct_IO on these pages, but as a
side effect prevents the frontend from using get_user_pages_fast on
them while they are being shared with the backend.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
---
 arch/x86/xen/p2m.c |   18 ++++++++++++++++++
 1 files changed, 18 insertions(+), 0 deletions(-)

diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 7ece122..c62ae5c 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -687,6 +687,7 @@ int m2p_add_override(unsigned long mfn, struct page *page,
        unsigned long uninitialized_var(address);
        unsigned level;
        pte_t *ptep = NULL;
+       int ret = 0;
 
        pfn = page_to_pfn(page);
        if (!PageHighMem(page)) {
@@ -722,6 +723,16 @@ int m2p_add_override(unsigned long mfn, struct page *page,
        list_add(&page->lru,  &m2p_overrides[mfn_hash(mfn)]);
        spin_unlock_irqrestore(&m2p_override_lock, flags);
 
+       /* p2m(m2p(mfn)) == mfn: the mfn is already present somewhere in
+        * this domain. Set the FOREIGN_FRAME_BIT in the p2m for the other
+        * pfn so that the following mfn_to_pfn(mfn) calls will return the
+        * pfn from the m2p_override (the backend pfn) instead.
+        * As a side effect GUPF might not be safe on the frontend pages
+        * while they are being shared with the backend. */
+       ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
+       if (ret >= 0 && get_phys_to_machine(pfn) == mfn)
+               set_phys_to_machine(pfn, FOREIGN_FRAME(mfn));
+
        return 0;
 }
 EXPORT_SYMBOL_GPL(m2p_add_override);
@@ -733,6 +744,7 @@ int m2p_remove_override(struct page *page, bool clear_pte)
        unsigned long uninitialized_var(address);
        unsigned level;
        pte_t *ptep = NULL;
+       int ret = 0;
 
        pfn = page_to_pfn(page);
        mfn = get_phys_to_machine(pfn);
@@ -802,6 +814,12 @@ int m2p_remove_override(struct page *page, bool clear_pte)
        } else
                set_phys_to_machine(pfn, page->index);
 
+       mfn &= ~FOREIGN_FRAME_BIT;
+       ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
+       if (ret >= 0 && get_phys_to_machine(pfn) == FOREIGN_FRAME(mfn) &&
+                       m2p_find_override(mfn) == NULL)
+               set_phys_to_machine(pfn, mfn);
+
        return 0;
 }
 EXPORT_SYMBOL_GPL(m2p_remove_override);
-- 
1.7.2.5


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