On Tue, May 07, 2019 at 10:08:50AM -0700, Linus Torvalds wrote: > On Tue, May 7, 2019 at 9:34 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > Would you consider my approach later on, under the guise of unification? > > WHY? > > The *only* advantage of your patch is that trivial "look up kernel stack" macro. > > Seriously. There's absolutely nothing else. > > Is that macro ugly? Yes. But it's directly explainable by just > pointing to the architecture documentation. > > It's a one-liner hack. > > And for that, you want to complicate the x86-32 entry and exit code? > > Do we have different emulation for "push" on 32-bit and 64-bit? Yes. > But again, that's just how the hardware works. This is not some > "generic hw-independent code". This is literally emulating > instructions that care about instruction encoding and bit size > details, where there are certainly _similarities_ (and in the case of > 'call', they look bit-identical), but it's also not like "same code" > is a big argument. That's why we have a helper function, to hide the > details. > > I point to my diffstat once again. It's smaller, and I argue that it > is actually conceptually *simpler* to simply say "this is how the > architecture works". > > And yes, I realize that I may be biased by the fact that I simply know > i386 so well, so to me it simply makes more sense to just work with > what the hardware gives us. The i386 exception model with the kernel > stack nesting is a *hell* of a lot simpler than the x86-64 one. The > fact is, x86-64 messed things up, and swapgs and friends are an > abomination against God. > > So the whole "let's clean up x86-32 to look like x86-64, which got > things right" is to me a completely bogus argument. x86-64 got the > "yes, push ss/sp unconditionally" part right, but got a lot of other > things horribly wrong. So this is all just one small detail that > differs, across two architectures that are similar but have very > different warts. > > But that diffstat is still hard, cold, unbiased data. regs->sp is *undefined* on x86-32. We're damning our future selves to have to always remember to use that darn kernel_stack_pointer() helper for eternity just because of x86-32. There have already been several bugs related to that. Because regs->sp is there, so why wouldn't you use it? If we truly want the code to reflect the HW, then we should have a pt_regs_kernel and a pt_regs_user on 32-bit. I'm pretty sure we don't want to go there... IMO, we either need to make the pt_regs struct(s) match the HW behavior, or make entry code match pt_regs. But this in-between thing just creates a bunch of headaches. -- Josh
