Re: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault

Xiao Guangrong
2012-05-02T05:40:04Z

On 04/29/2012 04:50 PM, Takuya Yoshikawa wrote:

> On Fri, 27 Apr 2012 11:52:13 -0300
> Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
>> Yes but the objective you are aiming for is to read and write sptes
>> without mmu_lock. That is, i am not talking about this patch.
>> Please read carefully the two examples i gave (separated by "example)").
>
> The real objective is not still clear.
>
> The ~10% improvement reported before was on macro benchmarks during live
> migration. At least, that optimization was the initial objective.
>
> But at some point, the objective suddenly changed to "lock-less" without
> understanding what introduced the original improvement.
>
> Was the problem really mmu_lock contention?
>

Takuya, i am so tired to argue the advantage of lockless write-protect
and lockless O(1) dirty-log again and again.

> If the path being introduced by this patch is really fast, isn't it
> possible to achieve the same improvement still using mmu_lock?
>
>
> Note: During live migration, the fact that the guest gets faulted is
> itself a limitation. We could easily see noticeable slowdown of a
> program even if it runs only between two GET_DIRTY_LOGs.
>

Obviously no.

It depends on what the guest is doing, from my autotest test, it very
easily to see that, the huge improvement is on bench-migration not
pure-migration.

>
>> The rules for code under mmu_lock should be:
>>
>> 1) Spte updates under mmu lock must always be atomic and
>> with locked instructions.
>> 2) Spte values must be read once, and appropriate action
>> must be taken when writing them back in case their value
>> has changed (remote TLB flush might be required).
>
> Although I am not certain about what will be really needed in the
> final form, if this kind of maybe-needed-overhead is going to be
> added little by little, I worry about possible regression.

Well, will you suggest Linus to reject all patches and stop
all discussion for the "possible regression" reason?