On 06/19/2013 08:41 PM, Paolo Bonzini wrote: > Il 19/06/2013 14:25, Xiao Guangrong ha scritto: >> On 06/19/2013 07:55 PM, Paolo Bonzini wrote: >>> Il 19/06/2013 13:53, Xiao Guangrong ha scritto: >>>> On 06/19/2013 07:32 PM, Paolo Bonzini wrote: >>>>> Il 19/06/2013 11:09, Xiao Guangrong ha scritto: >>>>>> Document it to Documentation/virtual/kvm/mmu.txt >>>>> >>>>> While reviewing the docs, I looked at the code. >>>>> >>>>> Why can't this happen? >>>>> >>>>> CPU 1: __get_spte_lockless CPU 2: __update_clear_spte_slow >>>>> ------------------------------------------------------------------------------ >>>>> write low >>>>> read count >>>>> read low >>>>> read high >>>>> write high >>>>> check low and count >>>>> update count >>>>> >>>>> The check passes, but CPU 1 read a "torn" SPTE. >>>> >>>> In this case, CPU 1 will read the "new low bits" and the "old high bits", right? >>>> the P bit in the low bits is cleared when do __update_clear_spte_slow, i.e, it is >>>> not present, so the whole value is ignored. >>> >>> Indeed that's what the comment says, too. But then why do we need the >>> count at all? The spte that is read is exactly the same before and >>> after the count is updated. >> >> In order to detect repeatedly marking spte present to stop the lockless side >> to see present to present change, otherwise, we can get this: >> >> Say spte = 0xa11110001 (high 32bits = 0xa, low 32bit = 0x11110001) >> >> CPU 1: __get_spte_lockless CPU 2: __update_clear_spte_slow >> ---------------------------------------------------------------------- >> read low: low= 0x11110001 >> clear the spte, then spte = 0x0ull >> read high: high = 0x0 >> set spte to 0xb11110001 (high 32bits = 0xb, >> low 32bit = 0x11110001) >> >> read low: 0x11110001 and see >> it is not changed. >> >> In this case, CPU 1 see the low bits are not changed, then it tries to access the memory at: >> 0x11110000. > > Got it. What about this in the comment to __get_spte_lockless: > > * The idea using the light way get the spte on x86_32 guest is from > * gup_get_pte(arch/x86/mm/gup.c). > * > * An spte tlb flush may be pending, because kvm_set_pte_rmapp > * coalesces them and we are running out of the MMU lock. Therefore > * we need to protect against in-progress updates of the spte. > * > * A race on changing present->non-present may get the old value for > * the high part of the spte. This is okay because the high part of > * the spte is ignored for non-present spte. > * > * However, we must detect a present->present change and reread the > * spte in case the change is in progress. Because all such changes > * are done in two steps (present->non-present and non-present->present), > * it is enough to count the number of present->non-present updates, > * which is done using clear_spte_count. It is fantastic :) -- 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
