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

[Takuya Yoshikawa <takuya.yoshikawa@gmail.com>]

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?

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.


> 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.

Thanks,
	Takuya