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

[Mark Cave-Ayland]

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.