Re: [RFC 0/3] x86: Patchable constants

[H. Peter Anvin]

On 02/07/18 09:01, Linus Torvalds wrote:
> 
> Look - much smaller code, and register %rcx isn't used at all. And no
> D$ miss on loading that constant (that is a constant depending on
> boot-time setup only).
> 
> It's rather more complex, but it actually gives a much bigger win. The
> code itself will be much better, and smaller.
> 
> The *infrastructure* for the code gets pretty hairy, though.
> 
> The good news is that the patch already existed to at least _some_
> degree. Peter Anvin did it about 18 months ago.
> 
> It was not really pursued all the way because it *is* a lot of extra
> complexity, and I think there was some other hold-up, but he did have
> skeleton code for the actual replacement.
> 
> There was a thread on the x86 arch list with the subject line
> 
>     Disgusting pseudo-self-modifying code idea: "variable constants"
> 
> but I'm unable to actually find the patch. I know there was at least a
> vert early prototype.
> 
> Adding hpa to the cc in the hope that he has some prototype code still
> laying around..
> 

The patchset I have is about 85% complete.  It mostly needs cleanup,
testing, and breaking into reasonable chunks (it got put on the
backburner for somewhat obvious reasons, but I don't think it'll take
very long at all to productize it.)

The main reason I haven't submitted it yet is that I got a bit overly
ambitious and wanted to implement a whole bunch of more complex
subcases, such as 64-bit shifts on a 32-bit kernel.  The win in that
case is actually quite huge, but it is requires data-dependent code
patching and not just immediate patching, which requires augmentation of
the alternatives framework.

	-hpa