Re: [RFC PATCH] powerpc/64s/radix: introduce option to disable broadcast tlbie

[Nicholas Piggin <npiggin@gmail.com>]

Christophe Leroy's on July 31, 2019 11:56 pm:
> 
> 
> Le 31/07/2019 à 14:32, Nicholas Piggin a écrit :
>> This is an initial hack of a quick option to disable use of the tlbie
>> instruction. This takes the simplest possible initial pass of just
>> replacing low level tlbie functions with IPIs. This means it's not as
>> performant as it could be if we spend some time optmizing it, but on
>> the other hand having a 1:1 replacement of tlbie is simple and can be
>> useful for comparisons so I think it's the right initial approach.
> 
> Can you explain why we want to optionnaly disable use of tlbie ?

It's something we've wanted to have more control of for a while.

One is for testing performance, especially on large systems it is not
always best to use tlbie. For example if you have threaded apps that
only run on a few CPUs (e.g., process * thread hybrid process model
server).

There is also concern about coherent accelerators may have high
latency to respond to tlbie, which can block the resource for others.

And we did have a hardware errata with early POWER9 it would have
been nice to disable it until the kernel could be updated with the
workaround.

Thanks,
Nick