Re: Question about rdhwr emulation.

[Ralf Baechle <ralf@linux-mips.org>]

On Wed, Oct 22, 2008 at 02:35:21AM +0100, Maciej W. Rozycki wrote:

>  This probe is necessary, because for a VIVT I-cache, code from there may 
> be executed even if there is no mapping stored for the virtual address of 
> the instruction in the TLB anymore.  However this trap handler wants to 
> read the instruction word from the memory and obviously this goes through 
> the D-cache which is not virtually tagged.  As such a TLB refill exception 
> would happen if the mapping was indeed absent.
> 
>  However, please note that this piece of code runs at the exception level 
> and therefore such a scenario would qualify as a nested exception.  Which 
> means the general exception vector would be used and the TLBL or TLBS 
> handler invoked as appropriate.  Neither of which are currently prepared 
> to do a refill.  Changing that would be rather trivial as it boils down to 
> checking the value of cp0.index.p and executiong TLBWR rather than TLBWI 
> as usual, but that is in the fast path, so we do not want to waste cycles 
> for such a corner case as RDHWR emulation.

To clarify, this behaviour of hitting in an VIVT I-cache even though there
is no address translation in the TLB is allowed but not required by the
the MIPS architecture spec.  From a software perspective it's a bit
quirky but it allows faster pipeline implementations.  The currently
supported VIVT I-cache processors are the SB1, 20K and 25K.  The SB1
has this behaviour; of the 20K and 25K I don't know.

  Ralf