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 <gleb@xxxxxxxxxx>
diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
index 891eb6d..a7b24cf 100644
--- a/arch/x86/kvm/paging_tmpl.h
+++ b/arch/x86/kvm/paging_tmpl.h
@@ -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.
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