On 12/16/2016 11:23 PM, Paolo Bonzini wrote:
On 16/12/2016 14:04, Xiao Guangrong wrote:
+ /*
+ * #PF can be fast if:
+ * 1. The shadow page table entry is not present, which could mean that
+ * the fault is potentially caused by access tracking (if enabled).
+ * 2. The shadow page table entry is present and the fault
+ * is caused by write-protect, that means we just need change the W
+ * bit of the spte which can be done out of mmu-lock.
+ *
+ * However, if access tracking is disabled we know that a non-present
+ * page must be a genuine page fault where we have to create a new SPTE.
+ * So, if access tracking is disabled, we return true only for write
+ * accesses to a present page.
+ */
+
+ return shadow_acc_track_mask != 0 ||
+ ((error_code & (PFERR_WRITE_MASK | PFERR_PRESENT_MASK))
+ == (PFERR_WRITE_MASK | PFERR_PRESENT_MASK));
acc-track can not fix a WRITE-access, this should be:
!(error_code & (PFERR_WRITE_MASK)) && shadow_acc_track_mask != 0 || ...
Access tracking makes pages non-present, so a !W !P fault can sometimes
be fixed.
One possibility is to test is_access_track_pte, but it is handled a
little below the call to page_fault_can_be_fast:
remove_acc_track = is_access_track_spte(spte);
/* Verify that the fault can be handled in the fast path */
if (!remove_acc_track && !remove_write_prot)
break;
It's not different from the way page_fault_can_be_fast return true for
writes, even if spte_can_locklessly_be_made_writable will return false
later.
So I think Junaid's patch is okay.
Yes, it is workable.
My suggestion is just a optimization. Figure out Write access which can
not be fixed by acc-track earlier in page_fault_can_be_fast() can stop
useless lockless-ly page-table walking.
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