Re: [PATCH 0/7] MIPS: Add extended ASID support

[James Hogan <james.hogan@imgtec.com>]

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On Mon, May 09, 2016 at 06:01:27PM +0100, Maciej W. Rozycki wrote:
> On Mon, 9 May 2016, Ralf Baechle wrote:
> 
> > Already PMC-Sierra's RM9000 / E9000 core had an extended ASID field, of
> > 12 bits for 4096 ASID contexts.  Afaics this was an extension derived
> > in-house back in the wild days before everything had to be sanctioned by
> > the architecture folks, so there is nothing in a config register to test
> > for it.
> 
>  Couldn't you just probe it in EntryHi directly, by writing all-ones, 
> reading back and seeing how many bits have stuck?

Note, the tlbinv feature in recent versions of MIPS32/MIPS64 arch has
EHINV bit in bit 10 (if I remember right) of EntryHi, which marks whole
tlb entry as invalid, and the small pages feature (for 1k pages) extends
VPN field downwards to bit 11.

Cheers
James

> 
> > PMCS simply extended the ASID field to 12 bits; no of the EntryHi bits
> > which today would conflict doing so did exist back then.
> 
>  Especially as it was as simple like that -- bits 12:8 were hardwired to 
> zeros in the usual implementations back then.
> 
>   Maciej

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