Re: [PATCH v3 04/15] KVM: MMU: flush tlb out of mmu lock when write-protect the sptes

Marcelo Tosatti
2013-11-19T00:19:58Z

On Fri, Nov 15, 2013 at 03:09:13PM +0800, Xiao Guangrong wrote:
> On 11/15/2013 02:39 AM, Marcelo Tosatti wrote:
> > On Thu, Nov 14, 2013 at 01:15:24PM +0800, Xiao Guangrong wrote:
> >>
> >> Hi Marcelo,
> >>
> >> On 11/14/2013 08:36 AM, Marcelo Tosatti wrote:
> >>
> >>>
> >>> Any code location which reads the writable bit in the spte and assumes if its not
> >>> set, that the translation which the spte refers to is not cached in a
> >>> remote CPU's TLB can become buggy. (*)
> >>>
> >>> It might be the case that now its not an issue, but its so subtle that
> >>> it should be improved.
> >>>
> >>> Can you add a fat comment on top of is_writeable_bit describing this?
> >>> (and explain why is_writable_pte users do not make an assumption
> >>> about (*).
> >>>
> >>> "Writeable bit of locklessly modifiable sptes might be cleared
> >>> but TLBs not flushed: so whenever reading locklessly modifiable sptes
> >>> you cannot assume TLBs are flushed".
> >>>
> >>> For example this one is unclear:
> >>>
> >>> if (!can_unsync && is_writable_pte(*sptep))
> >>> goto set_pte;
> >>> And:
> >>>
> >>> if (!is_writable_pte(spte) &&
> >>> !(pt_protect && spte_is_locklessly_modifiable(spte)))
> >>> return false;
> >>>
> >>> This is safe because get_dirty_log/kvm_mmu_slot_remove_write_access are
> >>> serialized by a single mutex (if there were two mutexes, it would not be
> >>> safe). Can you add an assert to both
> >>> kvm_mmu_slot_remove_write_access/kvm_vm_ioctl_get_dirty_log
> >>> for (slots_lock) is locked, and explain?
> >>>
> >>> So just improve the comments please, thanks (no need to resend whole
> >>> series).
> >>
> >> Thank you very much for your time to review it and really appreciate
> >> for you detailed the issue so clearly to me.
> >>
> >> I will do it on the top of this patchset or after it is merged
> >> (if it's possiable).
> >
> > Ok, can you explain why every individual caller of is_writable_pte have
> > no such assumption now? (the one mentioned above is not clear to me for
> > example, should explain all of them).
>
> Okay.

OK, thanks for checking.

> Generally speak, we 1) needn't care readonly spte too much since it
> can not be locklessly write-protected and 2) if is_writable_pte() is used
> to check mmu-mode's state we can check SPTE_MMU_WRITEABLE instead.
>
> There are the places is_writable_pte is used:
> 1) in spte_has_volatile_bits():
> 527 static bool spte_has_volatile_bits(u64 spte)
> 528 {
> 529 /*
> 530 * Always atomicly update spte if it can be updated
> 531 * out of mmu-lock, it can ensure dirty bit is not lost,
> 532 * also, it can help us to get a stable is_writable_pte()
> 533 * to ensure tlb flush is not missed.
> 534 */
> 535 if (spte_is_locklessly_modifiable(spte))
> 536 return true;
> 537
> 538 if (!shadow_accessed_mask)
> 539 return false;
> 540
> 541 if (!is_shadow_present_pte(spte))
> 542 return false;
> 543
> 544 if ((spte & shadow_accessed_mask) &&
> 545 (!is_writable_pte(spte) || (spte & shadow_dirty_mask)))
> 546 return false;
> 547
> 548 return true;
> 549 }
>
> this path is not broken since any spte can be lockless modifiable will do
> lockless update (will always return 'true' in the line 536).
>
> 2): in mmu_spte_update()
> 594 /*
> 595 * For the spte updated out of mmu-lock is safe, since
> 596 * we always atomicly update it, see the comments in
> 597 * spte_has_volatile_bits().
> 598 */
> 599 if (spte_is_locklessly_modifiable(old_spte) &&
> 600 !is_writable_pte(new_spte))
> 601 ret = true;
>
> The new_spte is a temp value that can not be fetched by lockless
> write-protection and !is_writable_pte() is stable enough (can not be
> locklessly write-protected).
>
> 3) in spte_write_protect()
> 1368 if (!is_writable_pte(spte) &&
> 1369 !spte_is_locklessly_modifiable(spte))
> 1370 return false;
> 1371
>
> It always do write-protection if the spte is lockelss modifiable.
> (This code is the aspect after applying the whole pachset, the code is safe too
> before patch "[PATCH v3 14/15] KVM: MMU: clean up spte_write_protect" since
> the lockless write-protection path is serialized by a single lock.).
>
> 4) in set_spte()
> 2690 /*
> 2691 * Optimization: for pte sync, if spte was writable the hash
> 2692 * lookup is unnecessary (and expensive). Write protection
> 2693 * is responsibility of mmu_get_page / kvm_sync_page.
> 2694 * Same reasoning can be applied to dirty page accounting.
> 2695 */
> 2696 if (!can_unsync && is_writable_pte(*sptep))
> 2697 goto set_pte;
>
> It is used for a optimization and the worst case is the optimization is disabled
> (walking the shadow pages in the hast table) when the spte has been locklessly
> write-protected. It does not hurt anything since it is a rare event. And the
> optimization can be back if we check SPTE_MMU_WRITEABLE instead.
>
> 5) fast_page_fault()
> 3110 /*
> 3111 * Check if it is a spurious fault caused by TLB lazily flushed.
> 3112 *
> 3113 * Need not check the access of upper level table entries since
> 3114 * they are always ACC_ALL.
> 3115 */
> 3116 if (is_writable_pte(spte)) {
> 3117 ret = true;
> 3118 goto exit;
> 3119 }
>
> Since kvm_vm_ioctl_get_dirty_log() firstly get-and-clear dirty-bitmap before
> do write-protect, the dirty-bitmap will be properly set again when fast_page_fault
> fix the spte who is write-protected by lockless write-protection.
>
> 6) in fast_page_fault's tracepoint:
> 244 #define __spte_satisfied(__spte) \
> 245 (__entry->retry && is_writable_pte(__entry->__spte))
> It causes the tracepoint reports the wrong result when fast_page_fault
> and tdp_page_fault/lockless-write-protect run concurrently, i guess it's
> okay since it's only used for trace.
>
> 7) in audit_write_protection():
> 202 if (is_writable_pte(*sptep))
> 203 audit_printk(kvm, "shadow page has writable "
> 204 "mappings: gfn %llx role %x\n",
> 205 sp->gfn, sp->role.word);
> It's okay since lockless-write-protection does not update the readonly sptes.
>
> >
> > OK to improve comments later.
>
> Thank you, Marcelo!
>