Re: [PATCH, rs6000] Allow libitm to use HTM on newer hw and kernels

[Segher Boessenkool <segher at kernel dot crashing dot org>]

On Wed, Dec 12, 2018 at 01:47:02PM -0600, Peter Bergner wrote:
> Libitm on POWER hardware looks for the "htm" bit in AT_HWCAP2 to determine
> whether it can use HTM when executing code within __transaction_atomic
> code blocks.  However, on newer hardware and kernels, the "htm" bit is no
> longer set and instead the "htm-no-suspend" bit is set, so we currently
> don't use HTM on new hw and kernels.  The following patch adds support
> for htm-no-suspend to libitm.  I have also added code to use the
> __builtin_cpu_supports() builtin if it is available, since that is
> much faster than using the getauxval libc call.
> 
> This passed bootstrap and regtesting with no errors and someone within
> IBM how had a POWER9 box with a newish kernel how ran into the problem
> confirmed it works for his test case.
> 
> Ok for mainline?  Should be backport this?

Okay for trunk (but see comment below).  I think it should be backported
yes, to 8 at least, probably 7.  Okay for those too.

> +/* This is a fairly new feature bit, so handle it not being defined.  */
> +#ifndef PPC_FEATURE2_HTM_NO_SUSPEND
> +# define PPC_FEATURE2_HTM_NO_SUSPEND 0
> +#endif

Doing it this way can be pretty surprising for users not aware you defined
it to 0.  Since there are no other users yet it's no big deal.


Segher