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