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

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Date:	Wed, 07 Sep 2011 12:09:15 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
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 10:41 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 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?

    J

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