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). OK to improve comments later. -- 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
