From: "H. Peter Anvin" Subject: Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out Date: Wed, 05 Oct 2011 17:17:09 -0700 Message-ID: <4E8CF385.2080804@zytor.com> References: <477dead9647029012f93c651f2892ed0e86b89e7.1317506051.git.jeremy.fitzhardinge@citrix.com> <20111003150205.GB2462@redhat.com> <4E89E28C.7010700@goop.org> <20111004141011.GA2520@redhat.com> <4E8B3489.60902@zytor.com> <4E8CF348.4080405@goop.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4E8CF348.4080405@goop.org> Sender: linux-kernel-owner@vger.kernel.org To: Jeremy Fitzhardinge Cc: Jason Baron , Steven Rostedt , "David S. Miller" , David Daney , Michael Ellerman , Jan Glauber , the arch/x86 maintainers , Xen Devel , Linux Kernel Mailing List , Jeremy Fitzhardinge , peterz@infradead.org List-Id: xen-devel@lists.xenproject.org On 10/05/2011 05:16 PM, Jeremy Fitzhardinge wrote: > On 10/04/2011 09:30 AM, H. Peter Anvin wrote: >> On 10/04/2011 07:10 AM, Jason Baron wrote: >>> 1) The jmp +0, is a 'safe' no-op that I know is going to initially >>> boot for all x86. I'm not sure if there is a 5-byte nop that works on >>> all x86 variants - but by using jmp +0, we make it much easier to debug >>> cases where we may be using broken no-ops. >>> >> There are *plenty*. jmp+0 is about as pessimal as you can get. > > As an aside, do you know if a 2-byte unconditional jmp is any more > efficient than 5-byte, aside from just being a smaller instruction and > taking less icache? > I don't know for sure, no. I probably depends on the CPU. -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.