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Date: Wed, 07 Sep 2011 12:09:15 -0700
From: Jeremy Fitzhardinge <jeremy@goop.org>
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To: Avi Kivity <avi@redhat.com>
CC: Don Zickus <dzickus@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@elte.hu>, the arch/x86 maintainers <x86@kernel.org>,
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        Nick Piggin <npiggin@kernel.dk>, Marcelo Tosatti <mtosatti@redhat.com>,
        KVM <kvm@vger.kernel.org>, Andi Kleen <andi@firstfloor.org>,
        Xen Devel <xen-devel@lists.xensource.com>,
        Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>,
        Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Subject: Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking
References: <1314996468.8255.0.camel@twins> <4E614FBD.2030509@goop.org> <20110906151408.GA7459@redhat.com> <4E66615E.8070806@goop.org> <20110906182758.GR5795@redhat.com> <4E66EF86.9070200@redhat.com> <20110907134411.GV5795@redhat.com> <4E678992.5050709@redhat.com> <20110907155657.GX5795@redhat.com> <4E679AF4.50209@redhat.com> <20110907165203.GQ6838@redhat.com> <4E67A551.4000502@redhat.com> <4E67A71A.5070903@goop.org> <4E67ACB6.40107@redhat.com>
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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