It was recently pointed out that most of the x86 architecture patches had been merged into the mainline 2.6.25 kernel, except for the kgdb patches. Linus Torvalds replied, "I won't even consider pulling it unless it's offered as a separate tree, not mixed up with other things. At that point I can give a look." He continued:
"That said, I explained to Ingo why I'm not particularly interested in it. I don't think that 'developer-centric' debugging is really even remotely our problem, and that I'm personally a lot more interested in infrastructure that helps normal users give better bug-reports. And kgdb isn't even _remotely_ it.
"So I'd merge a patch that puts oops information (or the whole console printout) in the Intel management stuff in a heartbeat. That code is likely much grottier than any kgdb thing will ever be (Intel really screwed up the interface and made it some insane XML thing), but it's also fundamentally more important - if it means that normal users can give oops reports after they happened in X (or, these days, probably more commonly during suspend/resume) and the machine just died."
Ingo Molnar summarized his pull request for changes to the x86 architecture bound for mainline inclusion in 2.6.25 noting, "it's not a small merge, it consists of 908 commits from 96 individual arch/x86 developers (!)". He continued, "a number of core files are changed as well: most notably percpu, debugging details, timers, the firewire remote debugging patch and ... the KGDB remote debugging stub in kernel/kgdb.c." He went on to detail the extent of the testing this tree has received, "in the past few weeks tens of thousands of random x86.git bzImages were successfully built and booted on a number of (commodity) 32-bit and
64-bit testsystems - and there has been a fair amount of test exposure on -mm as well." Regarding the remote kernel debugger, Ingo explained:
"We tested KGDB to be merge-worthy within the x86 architecture (the only supported architecture for now) and it's better to have kernel/kgdb.c than arch/x86/kernel/kgdb.c. The code is reasonably clean and the user-space exposure is small - the only real exposure is the decades-old remote GDB protocol. We are happy to fix up any further cleanliness comments that people might have - but we really wanted to start somewhere and get this thing moving. As an added bonus: finally a kernel debugger that can be read without puking too much ;-) [anyone remember KDB?]"
The final 2.6.24 Linux kernel is expected any day now, so the various subsystem maintainers have begun summarizing what changes are expected to be merged into the mainline kernel during the 2.6.25 merge window. Ingo Molnar spoke to changes for the x86 architecture, "there are 763 commits in x86.git so far, from more than 90 contributors, so it would be difficult to mention and credit every contribution in this mail." Along with a lengthy list of other changes, he included:
"Continued, intense arch/x86 unification and cleanup work by lots of people; FIFO ticket spinlocks for better spinlock scalability; 'regset' generalizations - the most important step towards utrace support (==next-gen ptrace); support for more than 255 CPUs [up to 4096 - in theory up to 65535]; almost complete 64-bit paravirt guest support; KGDB support on x86, finally!"
"This series kill the old i386 and x86_64 directories. The relevant files are moved and adapted and Kconfig.debug was consolidated (thanks to Randy)," Sam Ravnborg said, describing a set of 6 patches to finish the migration of physical files into the new x86 architecture directory. He described it as "a nice patch series that deletes more lines than it adds," going on to explain:
"I had to modify both the top-level Makefile and the kconfig Makefile to accomplish this. It was done in such a way that it is trivial for other archs to use the same mechanism should they have the need.
"To solve the defconfig issue (i386 and x86_64 cannot share one) the arch/x86/configs/ directory were introduced. This has been used by other archs for some time now but x86 had not had the need until now."
"Can we please finish up this merge a little more before we freeze 2.6.24? The way we currently have leftovers of arch/i386/ and arch/x86_64/ is quite a nightmare and not how the other architectures were merged," Christoph Hellwig asked, leading to an insightful reply by Ingo Molnar. Ingo began by noting, "to answer that question one should first be aware of the fundamental code quality problems that the unified x86 architecture has inherited from the split i386 and x86_64 architectures." He then utilized the
checkpatch script to generate a table of "coding style errors per one thousand lines of source code". In his table,
arch/i386/ rated 77.3 errors per thousand lines of source, with
arch/x86_64/ rating 96.0. The new unified
arch/x86/ rated a lower but still very high 74.1. He summarized, "it is plainly obvious that the x86_64 and i386 architectures were in a dreadful state of code quality before the unification. Their code quality was almost an order of magnitude worse than that of the core kernel (!) - and their code quality was significantly worse than that of a couple of other, comparable architectures." Ingo continued:
"So to answer your question: full unification is no easy task and it is not automatic at all. The x86_64 tree has diverged from the i386 tree in the past 5 years due to their illogical, forced separation and a resulting bitrot. The two architectures have grown different sets of cleanliness problems and different sets of functions with arbitrary differences that often cover the same functionality. It's all compounded by the fact that the 64-bit code is in worse shape than the 32-bit - so it's not like we could just pick the 64-bit code and use that as the unified code. The 32-bit code is also used about 8-10 times more frequently than the 64-bit code. So there is no easy 'just unify it' path."
Sam Ravnborg took a look at the x86 unification patches and commented, "from the mails and discussions I expected it be be obvious what was i386 only, what was shared and what was x86_64 only." He listed 16 files in
x86/pci and noted, "in the filename there is NOTHING for most of the non-shared code that tell that this file is used by only i386 or x86_64." Andi Kleen concurred, "exactly my point from KS. The big mash-up will not really make much difference in terms of Makefile clarity or whatever Thomas' point was. Apparently he wanted to eliminate a few lines of code from the Makefile and merge the header files completely?"
Linus Torvalds disagreed saying, "the problem right now is the *reverse* - even though they are in different subdirectories (and thus *look* like they are all separate), they aren't. So the merged end result is much better: at a first approximation everything is shared (largely true), and the ones that aren't shared can be made more obvious." He added, "at least things like "grep" will work sanely, and people will be *aware* that 'Oh, this touches a file that may be used by the other word-size'." Linus continued:
"Right now, we have people changing 'i386-only' files that turn out to be used by x86-64 too - through very subtle Makefile things that the person who only looks into the i386 Makefile will never even *see*.
"THAT is the problem (well, at least part of it)."
Thomas Gleixner described an effort to create a unified x86 architecture tree, "the core idea behind our project is simple to describe: we introduce a new arch/x86/ and include/asm-x86/ file hierarchy that includes all the existing 32-bit and 64-bit x86 code and allows the building of either a 32-bit (i386) kernel or a 64-bit (x86_64) kernel." Andi Kleen expressed some concern, "I think it's a bad idea because it means we can never get rid of any old junk. IMNSHO arch/x86_64 is significantly cleaner and simpler in many ways than arch/i386 and I would like to preserve that. Also in general arch/x86_64 is much easier to hack than arch/i386 because it's easier to regression test and in general has to care about much less junk. And I don't know of any way to ever fix that for i386 besides splitting the old stuff off completely." Additional concerns about legacy issues were countered by Linus Torvalds, "there really isn't that much legacy crud. There are things like random quirks, but every time I hear the (theoretical) argument about how much time and effort we save by having it duplicated somewhere else, I think about all the time we definitely waste by fixing the same bug twice (and worry about the cases where we don't)." Among the justifications for a unified architecture, Thomas noted:
"We believe that the whole x86 CPU family is very much related and should be supported in a single architecture tree. All 64-bit CPUs implement the ability to execute pure 32-bit kernels, and will probably do so for the next couple of decades. So it's not like it will ever be possible to get rid of our legacies: for example even the latest 64-bit CPUs implement the legacy 'A20 line' feature that was already a weird outdated hack in the days of 16-bit x8086 CPUs."
H. Peter Anvin submitted a series of patches rewriting the x86 setup code, "this patch set replaces the x86 setup code, which is currently all in assembly, with a version written in C, using the '.code16gcc' feature of binutils (which has been present since at least 2001.)" He went on to explain why he did this, "the new code is vastly easier to read, and, I hope, debug. It should be noted that I found a fair number of minor bugs while going through this code, and have attempted to correct them."
Linus Torvalds reacted favorably, "I can't really argue against this on any sane grounds - not only is it removing more lines than it adds, but moving from mostly unreadable assembly to C seems a good idea." He went on to note, "so let's just get this merged. But the question is, do we put it in 2.6.23-rc1, or do we put it in -mm for a few weeks, which would imply waiting for the next merge window? Andrew?" Andrew Morton pointed out that the patches have been in -mm already for a couple of months, "this code has been in -mm since 11 May, as git-newsetup.patch. It has caused (for what it is) astonishingly few problems. Maybe a couple of build glitches and one runtime failure, all quickly fixed. I'd say it's ready." Linus agreed, "Ok. That makes it easy. I'll just merge it."