Re: [PATCH] MIPS: Enable tlbinv and tlbinvf instructions for microMIPS; and mips32r3 and above

["Maciej W. Rozycki" <macro at imgtec dot com>]

On Fri, 25 Sep 2015, Andrew Bennett wrote:

> Currently the tlbinv and tlbinvf instructions are only enabled if the user 
> enables the EVA ASE.  There are two issues with this.  Firstly these 
> instructions are now in the Virtualization, not the EVA ASE (please see:
> http://imgtec.com/mips/architectures/virtualization/).  Secondly they can
> be optionally supported in MIPS ISA mips32r3 and above (in mips32r6 they
> are supported by default).  This patch addresses these issues by:
> 
> 1. Enabling the tlbinv and tlbinvf instructions by default for microMIPS 
>    (because microMIPS is also mips32r3).
> 
> 2. Enabling the tlbinv and tlbinvf instructions for MIPS when the ISA
>    is mips32r3 and above, or if the user enabled the EVA or Virtualization
>    ASEs.

 FWIW I think this approach is dangerous.  The rationale behind ISA/ASE 
options is to safety-guard code produced against hardware capabilities.  

 If I specify, say, `-march=mips32r3' (rather than e.g. `-march=cpu-foo'), 
then I want to be sure that code produced will run on any MIPS32r3 
processor.  And if a feature is optional, then undoubtedly there will be 
cases where a piece of hardware is missing it.  I want to be made aware of 
it at the build time rather than at the run time two years on when a piece 
of binary fails.

 So given my understanding of the availability of these instructions the 
following arrangements seem viable to me:

1. `-march=mips32r2' or lower: no way to enable these instructions other 
   than via a temporary `.set' override in source code,

2. `-march=mips32r3' to `-march=mips32r5': these instructions are disabled
   by default and can be enabled with `-meva', `-mvirt' or `-mtlbinv',

3. `-march=mips32r6' or higher: these instructions are unconditionally 
   enabled.

Of course any individual processor specified with `-march=' instead can 
enable or disable these instructions based on its implementation.

  Maciej