On 02/15/2012 10:07 PM, Avi Kivity wrote:
> On 02/15/2012 01:37 PM, Xiao Guangrong wrote:
>>>>
>>>> I would really like to move the IPI back out of the lock.
>>>>
>>>> How about something like a sequence lock:
>>>>
>>>>
>>>> spin_lock(mmu_lock)
>>>> need_flush = write_protect_stuff();
>>>> atomic_add(kvm->want_flush_counter, need_flush);
>>>> spin_unlock(mmu_lock);
>>>>
>>>> while ((done = atomic_read(kvm->done_flush_counter)) < (want =
>>>> atomic_read(kvm->want_flush_counter)) {
>>>> kvm_make_request(flush)
>>>> atomic_cmpxchg(kvm->done_flush_counter, done, want)
>>>> }
>>>>
>>>> This (or maybe a corrected and optimized version) ensures that any
>>>> need_flush cannot pass the while () barrier, no matter which thread
>>>> encounters it first. However it violates the "do not invent new locking
>>>> techniques" commandment. Can we map it to some existing method?
>>>
>>> There is no need to advance 'want' in the loop. So we could do
>>>
>>> /* must call with mmu_lock held */
>>> void kvm_mmu_defer_remote_flush(kvm, need_flush)
>>> {
>>> if (need_flush)
>>> ++kvm->flush_counter.want;
>>> }
>>>
>>> /* may call without mmu_lock */
>>> void kvm_mmu_commit_remote_flush(kvm)
>>> {
>>> want = ACCESS_ONCE(kvm->flush_counter.want)
>>> while ((done = atomic_read(kvm->flush_counter.done) < want) {
>>> kvm_make_request(flush)
>>> atomic_cmpxchg(kvm->flush_counter.done, done, want)
>>> }
>>> }
>>>
>>
>>
>> Hmm, we already have kvm->tlbs_dirty, so, we can do it like this:
>>
>> #define SPTE_INVALID_UNCLEAN (1 << 63 )
>>
>> in invalid page path:
>> lock mmu_lock
>> if (spte is invalid)
>> kvm->tlbs_dirty |= SPTE_INVALID_UNCLEAN;
>> need_tlb_flush = kvm->tlbs_dirty;
>> unlock mmu_lock
>> if (need_tlb_flush)
>> kvm_flush_remote_tlbs()
>>
>> And in page write-protected path:
>> lock mmu_lock
>> if (it has spte change to readonly |
>> kvm->tlbs_dirty & SPTE_INVALID_UNCLEAN)
>> kvm_flush_remote_tlbs()
>> unlock mmu_lock
>>
>> How about this?
>
> Well, it still has flushes inside the lock. And it seems to be more
> complicated, but maybe that's because I thought of my idea and didn't
> fully grok yours yet.
>
Oh, i was not think that flush all tlbs out of mmu-lock, just invalid page
path.
But, there still have some paths need flush tlbs inside of mmu-lock(like
sync_children, get_page).
In your code:
>>> /* must call with mmu_lock held */
>>> void kvm_mmu_defer_remote_flush(kvm, need_flush)
>>> {
>>> if (need_flush)
>>> ++kvm->flush_counter.want;
>>> }
>>>
>>> /* may call without mmu_lock */
>>> void kvm_mmu_commit_remote_flush(kvm)
>>> {
>>> want = ACCESS_ONCE(kvm->flush_counter.want)
>>> while ((done = atomic_read(kvm->flush_counter.done) < want) {
>>> kvm_make_request(flush)
>>> atomic_cmpxchg(kvm->flush_counter.done, done, want)
>>> }
>>> }
I think we do not need handle all tlb-flushed request here since all of these
request can be delayed to the point where mmu-lock is released , we can simply
do it:
void kvm_mmu_defer_remote_flush(kvm, need_flush)
{
if (need_flush)
++kvm->tlbs_dirty;
}
void kvm_mmu_commit_remote_flush(struct kvm *kvm)
{
int dirty_count = kvm->tlbs_dirty;
smp_mb();
if (!dirty_count)
return;
if (make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
++kvm->stat.remote_tlb_flush;
cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);
}
if this is ok, we only need do small change in the current code, since
kvm_mmu_commit_remote_flush is very similar with kvm_flush_remote_tlbs().
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