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KVM: MMU: simplify folding of dirty bit into accessed_dirty

PATCH Applied Gleb Natapov 2 participants 2 messages Dec 27, 2012 – Jan 7, 2013

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x86 KVM KVM x86
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Gleb Natapov

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#0 lore raw

PATCH 1/1 KVM: MMU: simplify folding of dirty bit into accessed_dirty

MMU code tries to avoid if()s HW is not able to predict reliably by using bitwise operation to streamline code execution, but in case of a dirty bit folding this gives us nothing since write_fault is checked right before the folding code. Lets just piggyback onto the if() to make code more clear.

Signed-off-by Gleb Natapov <[email protected]>
arch/x86/kvm/paging_tmpl.h +6 -10
index 891eb6d..a7b24cf 100644
@@ -249,16 +249,12 @@ retry_walk:
 
 	if (!write_fault)
 		protect_clean_gpte(&pte_access, pte);
-
-	/*
-	 * On a write fault, fold the dirty bit into accessed_dirty by shifting it one
-	 * place right.
-	 *
-	 * On a read fault, do nothing.
-	 */
-	shift = write_fault >> ilog2(PFERR_WRITE_MASK);
-	shift *= PT_DIRTY_SHIFT - PT_ACCESSED_SHIFT;
-	accessed_dirty &= pte >> shift;
+	else
+		/*
+		 * On a write fault, fold the dirty bit into accessed_dirty by
+		 * shifting it one place right.
+		 */
+		accessed_dirty &= pte >> (PT_DIRTY_SHIFT - PT_ACCESSED_SHIFT);
 
 	if (unlikely(!accessed_dirty)) {
 		ret = FNAME(update_accessed_dirty_bits)(vcpu, mmu, walker, write_fault);

--
Gleb.

Applied
#1 lore raw
Quoted 6 lines · Gleb Natapov

MMU code tries to avoid if()s HW is not able to predict reliably by using bitwise operation to streamline code execution, but in case of a dirty bit folding this gives us nothing since write_fault is checked right before the folding code. Lets just piggyback onto the if() to make code more clear.

Signed-off-by Gleb Natapov <[email protected]>

Applied, thanks.