[Xen-devel] Re: [PATCH RFC V5 00/11] Paravirtualized ticketlocks

["H. Peter Anvin" <hpa@xxxxxxxxx>]

On 10/13/2011 09:44 AM, Jeremy Fitzhardinge wrote:
> 
> Yeah, that's a good question.  There are three mechanisms with somewhat
> overlapping concerns:
> 
>   * alternative()
>   * pvops patching
>   * jump_labels
> 
> Alternative() is for low-level instruction substitution, and really only
> makes sense at the assembler level with one or two instructions.
> 
> pvops is basically a collection of ordinary _ops structures full of
> function pointers, but it has a layer of patching to help optimise it. 
> In the common case, this just replaces an indirect call with a direct
> one, but in some special cases it can inline code.  This is used for
> small, extremely performance-critical things like cli/sti, but it
> awkward to use in general because you have to specify the inlined code
> as a parameterless asm.
> 
> Jump_labels is basically an efficient way of doing conditionals
> predicated on rarely-changed booleans - so it's similar to pvops in that
> it is effectively a very ordinary C construct optimised by dynamic code
> patching.

Then there is static_cpu_has(), which is basically jump labels
implemented using the alternatives mechanism.

If nothing else it would be good to:

1. Make more general use of ops patching;
2. Merge mechanisms where practical.

        -hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.


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