Even though the Wii's official library is mostly set, both the GameCube and Wii are entering a new golden age as a popular environment for randomizers, full-game mods, incredible cheat codes, and much more. Stalwarts like the Super Smash Bros. Brawl Mod, Project M have been around for years, but now there are many other communities around various games breathing new life into them. You can find codes to help balance games like Mario Party 5, content mods for Kirby Air Ride that add tons of new rides and hundreds of songs, and trackpacks for Mario Kart Wii that add hundreds of custom tracks to the game. Wiimmfi's also provides their own backup Wi-Fi servers for many unmodified games and their Mario Kart mods!
While most of these mods can be enjoyed on a hacked Wii, many users rely on Dolphin in order to play them. Emulating these mods can be quite the challenge, as they often will do things in ways that game developers would not. Assumptions that Dolphin makes can often be broken and certain features that mod developers use can be extremely slow or downright unreasonable to emulate. In the case of Wiimmfi's Mario Kart Fun Packs, the mod creators have put in work over the years to improve their experience in Dolphin and even support emulated users playing alongside console users online... so long as you're willing to dump and use your Wii's NAND. Earlier this month, a slight change to Wiimmfi's online networking broke Dolphin support without affecting real Wii Consoles. Not wanting to leave their emulated users high and dry, they reported the bug to us.
delroth quickly took up the mantle of investigating the bug with assistance from the Wiimmfi team. Within a few hours, the cooperation paid off as the list of probable causes was narrowed down to one annoying feature: The Instruction Cache. Dolphin pretty much has no ability to emulate the GC/Wii CPU data cache and likely never will due to the performance implications, but Dolphin does have some ability to emulate the instruction cache, though it's best to avoid testing the emulator. This is normally not a problem with retail games because it's rather bad form for a game to rely excessively on cache quirks, unless they were intentionally trying to break an emulator. There are occasionally games that inadvertently rely on cache behavior, that's something to tackle on another day. Dolphin's emulation of the instruction cache is normally good enough and almost nothing relies on data cache.
Mods are different; developers are usually working on a blackbox and don't have the same level of familiarity with the hardware. Unless they specifically tested codes on both Dolphin and Wii, there's a chance they wouldn't even know something was broken. There have been many issues reported around mods that, while Dolphin is at fault, we really don't have any recourse for the users afflicted. If a mod doesn't care about running on Dolphin and uses dcache or perhaps another annoying feature, there isn't much we can do but shrug it off.
In the case of Wiimmfi's server, through cooperation from both sides, we were able to find the cache coherency issue and fix it serverside! Users who already have the latest version of the mod don't have to do anything except try to connect. If you're looking for a more detailed explanation of what was going wrong (as it's rather interesting,) you can find delroth's full writeup on the issue tracker.
In order to track down behavior like this in the future, delroth also added game quirk reporting to Dolphin's data collection service, so Dolphin will now automatically let us know what games are instruction cache sensitive in the manner that broke this particular mod. With that, we also have a lot of other exciting changes this month, so now it's time to dive into this month's notable changes!