Re: [PATCH 2/4, v2] x86: enlightenment for ticket spin locks - Xen implementation

["H. Peter Anvin" <hpa@zytor.com>]

On 06/30/2010 06:23 AM, Jeremy Fitzhardinge wrote:
> 
> pvops is a superset of alternative instruction patching, and are really
> designed to serve different purposes.  There are some areas in which
> there's some overlap, but otherwise they are distinct.  In particular,
> alternative instructions are really only useful if you can express the
> patch in terms of the presence or absence of a particular cpu feature. 
> It can't do multi-way choice, and it can't do anything other than insert
> literal instructions.  pvops patching can do multi-way, and has a
> higher-level view of each patch site which allows it to do things like
> generate appropraite save/restores, make inline vs call decisions, nop
> out nop callsites, etc.
> 

A lot of this -- in particular the multiway -- is a defect in the
alternatives implementation and should have been addressed as such.  One
of the biggest problems with pvops as it currently stands is that it is
monolithic; in general we have this class of problems (static selection)
in a *lot* more places than we're dealing with right now, and as such,
generalizing *something* -- be it pvops or alternatives -- would be useful.

gcc 4.5 also includes a very powerful facility called "asm goto", which
I have already used to implement static_cpu_has().  Again, that
particular construct (unlike "asm goto" itself) doesn't support multiway.

	-hpa