Re: [RFC PATCH 04/14] sparc64: load shared id into context register 1

[David Miller <davem@davemloft.net>]

From: Mike Kravetz <mike.kravetz@oracle.com>
Date: Sun, 18 Dec 2016 16:06:01 -0800

> Ok, let me try to find a way to eliminate these loads unless the application
> is using shared context.
> 
> Part of the issue is a 'backwards compatibility' feature of the processor
> which loads/overwrites register 1 every time register 0 is loaded.  Somewhere
> in the evolution of the processor, a feature was added so that register 0
> could be loaded without overwriting register 1.  That could be used to
> eliminate the extra load in some/many cases.  But, that would likely lead
> to more runtime kernel patching based on processor level.  And, I don't
> really want to add more of that if possible.  Or, perhaps we only enable
> the shared context ID feature on processors which have the ability to work
> around the backwards compatibility feature.

Until the first process uses shared mappings, you should not touch the
context 1 register in any way for any reason at all.

And even once a process _does_ use shared mappings, you only need to
access the context 1 register in 2 cases:

1) TLB processing for the processes using shared mappings.

2) Context switch MMU state handling, where either the previous or
   next process is using shared mappings.