Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking

[Avi Kivity]

On 09/07/2011 08:17 PM, Jeremy Fitzhardinge wrote:
> On 09/07/2011 10:09 AM, Avi Kivity wrote:
> >  On 09/07/2011 07:52 PM, Don Zickus wrote:
> >>  >
> >>  >   May I ask how?  Detecting a back-to-back NMI?
> >>
> >>  Pretty boring actually.  Currently we execute an NMI handler until
> >>  one of
> >>  them returns handled.  Then we stop.  This may cause us to miss an
> >>  NMI in
> >>  the case of multiple NMIs at once.  Now we are changing it to execute
> >>  _all_ the handlers to make sure we didn't miss one.
> >
> >  That's going to be pretty bad for kvm - those handlers become a lot
> >  more expensive since they involve reading MSRs.
>
> How often are you going to get NMIs in a kvm guest?

We'll soon have the perf-based watchdog firing every 60s worth of 
instructions or so.  But if we implement your new kick pvop using NMI 
then it can be _very_ often.

> >    Even worse if we start using NMIs as a wakeup for pv spinlocks as
> >  provided by this patchset.
>
> Hm, I'm interested to know what you're thinking in more detail.  Can you
> leave an NMI pending before you block in the same way you can with
> "sti;halt" with normal interrupts?

Nope.  But you can do

   if (regs->rip in critical section)
           regs->rip = after_halt;

and effectively emulate it.  The critical section is something like

    critical_section_start:
        if (woken_up)
            goto critical_section_end;
        hlt
    critical_section_end:

> I was thinking you might want to do something with monitor/mwait to
> implement the blocking/kick ops. (Handwave)

monitor/mwait are incredibly expensive to virtualize since they require 
write-protecting a page, IPIs flying everywhere and flushing tlbs, not 
to mention my lovely hugepages being broken up mercilessly.

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

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