File 22526-ept-get-entry-no-lock.patch of Package xen

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File 22526-ept-get-entry-no-lock.patch of Package xen (Revision 56f8bfe249b4ac767a99b22790a3090b)

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# HG changeset patch
# User Keir Fraser <keir@xen.org>
# Date 1292410025 0
# Node ID 7a5ee380041707177ca9c78e800095d1f5f3d373
# Parent  01f3b350902385627d1fa9e8cd1c231953e7610c
ept: Remove lock in ept_get_entry, replace with access-once semantics.

This mirrors the RVI/shadow situation, where p2m read access is
lockless because it's done in the hardware (linear map of the p2m
table).

This fixes the original bug (call it bug A) without introducing bug B
(a deadlock).

Bug A was caused by a race when updating p2m entries: between testing
if it's valid, and testing if it's populate-on-demand, it may have
been changed from populate-on-demand to valid.

My original patch simply introduced a lock into ept_get_entry, but
that caused bug B, caused by circular locking order: p2m_change_type
[grabs p2m lock] -> set_p2m_entry -> ept_set_entry ->
ept_set_middle_level -> p2m_alloc [grabs hap lock] write cr4 ->
hap_update_paging_modes [grabes hap lock] -> hap_update_cr3 ->
gfn_to_mfn -> ept_get_entry -> [grabs p2m lock]

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>

--- a/xen/arch/x86/mm/hap/p2m-ept.c
+++ b/xen/arch/x86/mm/hap/p2m-ept.c
@@ -395,10 +395,6 @@ static mfn_t ept_get_entry(struct domain
     int i;
     int ret = 0;
     mfn_t mfn = _mfn(INVALID_MFN);
-    int do_locking = !p2m_locked_by_me(d->arch.p2m);
-
-    if ( do_locking )
-        p2m_lock(d->arch.p2m);
 
     *t = p2m_mmio_dm;
 
@@ -476,8 +472,6 @@ static mfn_t ept_get_entry(struct domain
     }
 
 out:
-    if ( do_locking )
-        p2m_unlock(d->arch.p2m);
     unmap_domain_page(table);
     return mfn;
 }

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File 22526-ept-get-entry-no-lock.patch of Package xen (Revision 56f8bfe249b4ac767a99b22790a3090b)

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