Messages in this thread | | | Date | Wed, 05 Oct 2011 17:47:21 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out |
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On 10/05/2011 05:17 PM, H. Peter Anvin wrote: > 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.
I was thinking about making the jump-label stuff generate a small jmp if the offset is small (specifically "jmp; nop3", or perhaps "jmp; ud2a; nop" to make absolutely sure there's no speculation beyond the jmp), on the grounds that, while it probably doesn't matter for any modern Intel/AMD processor, it may help for others. But I couldn't find any concrete evidence to support it, and there's already enough questions about doing "live" code updates without adding more instruction patterns.
J
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