[OpenBIOS] SPARC64: Forth re-entrancy during MMU I/D TLB miss

[Blue Swirl]

On Wed, Sep 1, 2010 at 12:28 PM, Mark Cave-Ayland
<mark.cave-ayland at siriusit.co.uk> wrote:
> Blue Swirl wrote:
>
>>> So having a poke around various bits of documentation relating to
>>> implementing pgmap@, I've come to the realisation that since the Solaris
>>> kernel needs to install it's own va>tte-data implementation into OpenBIOS
>>> then it's going to be necessary to call back into Forth from the D/I-MMU
>>> miss traps.
>>
>> The thought of that gives me the shivers.
>
> It sounds scary, but I don't think it will be too bad. Looking at
> arch/sparc64/entry.S, it seems like the OpenBIOS memory regions are locked
> into the TLB as N x 512K entries - so as long as we don't end up with MMU
> miss trap recursion, we should be good.
>
> The plan I have in my mind goes something like this:
>
> - Alter the D/I-MMU trap miss handlers so they invoke a C function rather
> than iterate through the translations structure in ASM.

The registers need to be saved at this point. There are also several
sets of global registers (%g1-%g7), which may need separate handling.

> - Initially make the C function do exactly the same as the current ASM for
> testing.
>
> - Create a simple Forth function to perform the address translation.
>
> - Modify the C function to invoke feval(...) on the translation function
> instead of iterating through the translations structure directly.
>
>>> I've been looking around the code in arch/sparc64/switch.S but it looks
>>> as
>>> if this code only swaps between 2 individual contexts - am I right in
>>> thinking there needs to be a way of specifying the memory address of the
>>> context for this code to be reusable during a trap? Or would a better way
>>> be
>>> to duplicate this file but with another static context used only for MMU
>>> miss traps?
>>
>> Sparc64 should use the same approach as Sparc32 (and x86): save the
>> context to stack. It's even easier than Sparc32 because there is a
>> window flush instruction.
>
> Hmmm. SPARC64 definitely doesn't do this right at the moment - instead of
> saving context to the stack, it seems to save it back into the static
> context structure :(
>
> Looks like the first trick is to figure out how to store the context on the
> stack instead. However, since __switch_context automatically restores all
> registers upon exit, I'm not sure even then we could use it  within a trap
> handler since we'd want the TTE data entry returned in one of the registers
> when we are finished so we can add the entry to the MMU. Maybe there is a
> better way of refactoring this code so we can share the majority of it?

The saved context could be stored into a location which would be
visible to C/Forth code, so that it could alter the return value
register.

> Also I see that arch/sparc64/switch.S is marked as ".align 4" - shouldn't
> this be 8 for a 64-bit architecture? Not that it seems to matter at the
> moment...

The instructions on Sparc64 still only require 4 byte alignment. If we
also considered cache line effects, the alignment should be 32 or 64
bytes, like Linux .S files. In QEMU, the best alignment probably
depends on host data cache line size.