Re: Question about x86/mm/gup.c's use of disabled interrupts

[Avi Kivity]

Jeremy Fitzhardinge wrote:
> Avi Kivity wrote:
> > > Hm, awkward if flush_tlb_others doesn't IPI...
> >
> > How can it avoid flushing the tlb on cpu [01]?  It's it's 
> > gup_fast()ing a pte, it may as well load it into the tlb.
>
> xen_flush_tlb_others uses a hypercall rather than an IPI, so none of 
> the logic which depends on there being an IPI will work.

Right, of course, that's what we were talking about.  I thought 
optimizations to avoid IPIs if an mm never visited a cpu.

> > > Simplest fix is to make gup_get_pte() a pvop, but that does seem 
> > > like putting a red flag in front of an inner-loop hotspot, or 
> > > something...
> > >
> > > The per-cpu tlb-flush exclusion flag might really be the way to go.
> >
> > I don't see how it will work, without changing Xen to look at the flag?
> >
> > local_irq_disable() is used here to lock out a remote cpu, I don't 
> > see why deferring the flush helps.
>
> Well, no, not deferring.  Making xen_flush_tlb_others() spin waiting 
> for "doing_gup" to clear on the target cpu.  Or add an explicit notion 
> of a "pte update barrier" rather than implicitly relying on the tlb 
> IPI (which is extremely convenient when available...).

Pick up a percpu flag from all cpus and spin on each?  Nasty.

You could use the irq enabled flag; it's available and what native spins 
on (but also means I'll need to add one if I implement this).

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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