linux-kernel - Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

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Date:	Wed, 28 Sep 2011 16:55:24 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Stephan Diestelhorst" <stephan.diestelhorst@....com>,
	"Jeremy Fitzhardinge" <jeremy.fitzhardinge@...rix.com>,
	"Ingo Molnar" <mingo@...e.hu>, "Andi Kleen" <andi@...stfloor.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Nick Piggin" <npiggin@...nel.dk>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	<xen-devel@...ts.xensource.com>, "Avi Kivity" <avi@...hat.com>,
	"Marcelo Tosatti" <mtosatti@...hat.com>,
	"KVM" <kvm@...r.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized
	 ticketlocks

>>> On 28.09.11 at 17:38, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Tue, Sep 27, 2011 at 9:44 AM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>>
>> I guess it comes down to throwing myself on the efficiency of some kind
>> of fence instruction.  I guess an lfence would be sufficient; is that
>> any more efficient than a full mfence?  At least I can make it so that
>> its only present when pv ticket locks are actually in use, so it won't
>> affect the native case.
> 
> Please don't play with fences, just do the final "addb" as a locked 
> instruction.
> 
> In fact, don't even use an addb, this whole thing is disgusting:
> 
>   movzwl (%rdi),%esi (esi:=0x0400)
>   addb   $0x2,(%rdi) (LOCAL copy of lock is now: 0x0402)
>   movzwl (%rdi),%eax (local forwarding from previous store: eax := 0x0402)
> 
> just use "lock xaddw" there too.

I'm afraid that's not possible, as that might carry from the low 8 bits
into the upper 8 ones, which must be avoided.

Jan

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