Re: [Xen-ia64-devel] [RFC] grant table API clean up and performancetuning memo

[Tristan Gingold <Tristan.Gingold@xxxxxxxx>]

Le Jeudi 27 Avril 2006 04:31, Isaku Yamahata a écrit :
[...]
> > Two questions:
> > It seems your mechanism still need to trust the domains.  So we need to
> > add checks for itc/ptc.
> >
> > However, can a granted page be mapped several times ?
> > If so, tracking is necessary and has a cost.
>
> Grant table API allows it. Xen tracks it by the counter(act->pin).
> However current xenLinux maps one-time per a granted page, I believe.
Even if it is mapped only once in guest physical memory, it may still be 
mapped many times in virtual memory.  True ?

> > From my point of view and my experience, flushing all the tlb/vhpt is
> > really costly (and even more with SMP-g), but it is necessary.
>
> The cost is very high.
>
> > Currently, we *don't* flush tlb/vhpt after unmapping granted page.  This
> > really creates instability (at least of gcc segfaults).
>
> My understanding is as follows.
> On the current P=M model, only dom0 maps/unmap granted pages.
> Because Dom0 accesses a granted page via machine address (given that m=p),
> flush isn't necessary.
> But the bug is there, there must be something I miss.
From my early tests, stability increase when tlb is flushed after umapping.
But this may be a side effect.

> > I don't have yet a magic solution.  I think we need to fix gcc segfault
> > bug first, and then we need to improve performance.
>
> I fully agree with you. Bug fix first, tuning next.
I am currently working on this.

Tristan.



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