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

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Message-ID: <4E67ACB6.40107@redhat.com>
Date:	Wed, 07 Sep 2011 20:41:10 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Don Zickus <dzickus@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>, Andi Kleen <andi@...stfloor.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>
Subject: Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking

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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