[Xen-changelog] [xen master] x86: fix determination of bit count for struct domain allocations

[patchbot@xxxxxxx]

commit b3d2f8b2cba9fce5bc8995612d0d13fcefec7769
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Mar 24 10:48:03 2014 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Mar 24 10:48:03 2014 +0100

    x86: fix determination of bit count for struct domain allocations
    
    We can't just add in the hole shift value, as the hole may be at or
    above the 44-bit boundary. Instead we need to determine the total bit
    count until reaching 32 significant (not squashed out) bits in PFN
    representations.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Keir Fraser <keir@xxxxxxx>
---
 xen/arch/x86/domain.c |   27 ++++++++++++++++++++++++++-
 1 files changed, 26 insertions(+), 1 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 0d563de..b48f2dc 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -180,6 +180,28 @@ void dump_pageframe_info(struct domain *d)
     spin_unlock(&d->page_alloc_lock);
 }
 
+/*
+ * The hole may be at or above the 44-bit boundary, so we need to determine
+ * the total bit count until reaching 32 significant (not squashed out) bits
+ * in PFN representations.
+ * Note that the way "bits" gets initialized/updated/bounds-checked guarantees
+ * that the function will never return zero, and hence will never be called
+ * more than once (which is important due to it being deliberately placed in
+ * .init.text).
+ */
+static unsigned int __init noinline _domain_struct_bits(void)
+{
+    unsigned int bits = 32 + PAGE_SHIFT;
+    unsigned int sig = hweight32(~pfn_hole_mask);
+    unsigned int mask = pfn_hole_mask >> 32;
+
+    for ( ; bits < BITS_PER_LONG && sig < 32; ++bits, mask >>= 1 )
+        if ( !(mask & 1) )
+            ++sig;
+
+    return bits;
+}
+
 struct domain *alloc_domain_struct(void)
 {
     struct domain *d;
@@ -187,7 +209,10 @@ struct domain *alloc_domain_struct(void)
      * We pack the PDX of the domain structure into a 32-bit field within
      * the page_info structure. Hence the MEMF_bits() restriction.
      */
-    unsigned int bits = 32 + PAGE_SHIFT + pfn_pdx_hole_shift;
+    static unsigned int __read_mostly bits;
+
+    if ( unlikely(!bits) )
+         bits = _domain_struct_bits();
 
     BUILD_BUG_ON(sizeof(*d) > PAGE_SIZE);
     d = alloc_xenheap_pages(0, MEMF_bits(bits));
--
generated by git-patchbot for /home/xen/git/xen.git#master

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