Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

[Peter Zijlstra]

On Tue, Mar 01, 2016 at 01:47:38PM -0800, H. Peter Anvin wrote:
> On 03/01/16 13:32, Peter Zijlstra wrote:
> > On Tue, Mar 01, 2016 at 08:23:12PM +0000, Mathieu Desnoyers wrote:
> >> I think it's important that user-space fast-paths can quickly
> >> detect whether the feature is enabled without having to rely on
> >> always reading a separate cache-line. I've put together an ABI
> >> proposal that take into account the feedback received so far.
> > 
> > Nah, adding detectoring code to fast paths is silly, makes them less
> > fast. Doesn't userspace have self modifying code? I know that at least
> > glibc does linker trickery to call different functions depending on
> > runtime context.
> > 
> 
> No, userspace does not have self-modifying code.  The glibc indirect
> function is done at dynamic link time; it is also worth noting that
> resolving global symbols through dynamic linking often requires an
> indirect call.

Boy that blows. And here I was thinking you could edit the code at
dynamic link time because nobody was running it yet :/

And I suppose JITs need an (effective) munmap()+mmap() cycle to ensure
the 'old' code is flushed from all caches etc..?