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

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Date:	Thu, 08 Sep 2011 10:51:26 +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 10:09 PM, Jeremy Fitzhardinge wrote:
> On 09/07/2011 10:41 AM, Avi Kivity wrote:
> >>  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:
>
> Hm.  It's a pity you have to deliver an actual interrupt to implement
> the kick though.

I don't think it's that expensive, especially compared to the 
double-context-switch and vmexit of the spinner going to sleep.  On AMD 
we do have to take an extra vmexit (on IRET) though.

> >>
> >>  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.
>
> Or what about a futex-like hypercall?
>

Well we could have a specialized sleep/wakeup hypercall pair like Xen, 
but I'd like to avoid it if at all possible.

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

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