Re: [Qemu-devel] target-sparc has inverse cwp logic for SAVE/RESTORE?

[Olivier Danet]

On 18/05/2014 14:48, Mark Cave-Ayland wrote:
> Hi all,
> 
> I've been working on debugging a window-related OpenBIOS issue and noticed 
> that the cwp register logic in QEMU appears to be backwards according to the 
> SPARCv9 specification. From sections 6.3.6.1 and 6.3.6.2:
> 
> "The SAVE instruction allocates a new register window and saves the caller’s 
> register window by incrementing the CWP register."
> 
> "The RESTORE instruction restores the previous register window by 
> decrementing the CWP register."
> 
> In target-sparc/win_helper.c the logic in helper_save() and helper_restore() 
> is inverted, i.e. executing SAVE decrements cwp while executing RESTORE 
> increments cwp.
> 
> The surprise here was that executing SAVE when cwp == 0 changed cwp to 7 
> rather than 1. AFAICT there should be no functional difference, but it would 
> make things less confusing when debugging window traps if the logic from the 
> specification was followed. Does anyone know why this is currently done this 
> way?
> 
> 
> ATB,
> 
> Mark.
> 
The problem may be related to the fact that the 32bits SPARCv8 and 64bits 
SPARCv9 work in opposite directions !

SparcV9 standard, page 360/399 :
The SPARC-V9 CWP register is incremented during a SAVE instruction and 
decremented during
a RESTORE instruction. Although this is the opposite of PSR.CWP’s behavior in 
SPARC-V8, the
only software it should affect is a few trap handlers that operate in 
privileged mode, and that must
be rewritten for SPARC-V9 anyway. This change will have no effect on 
nonprivileged software.


Olivier