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

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

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.

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!