On 21/12/2016 10:49, Xiao Guangrong wrote: > > > 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. That optimization can be done as a follow-up, but your suggestion was not complete. page_fault_can_be_fast must be conservative and return true if unsure. Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
