On Mon, Mar 9, 2015 at 6:04 AM, Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx> wrote: > On Sat, Mar 7, 2015 at 9:37 AM, tip-bot for Andy Lutomirski > <tipbot@xxxxxxxxx> wrote: >> Commit-ID: a7fcf28d431ef70afaa91496e64e16dc51dccec4 >> Gitweb: http://git.kernel.org/tip/a7fcf28d431ef70afaa91496e64e16dc51dccec4 >> Author: Andy Lutomirski <luto@xxxxxxxxxxxxxx> >> AuthorDate: Fri, 6 Mar 2015 17:50:19 -0800 >> Committer: Ingo Molnar <mingo@xxxxxxxxxx> >> CommitDate: Sat, 7 Mar 2015 09:34:03 +0100 >> >> x86/asm/entry: Replace this_cpu_sp0() with current_top_of_stack() and fix it on x86_32 >> >> I broke 32-bit kernels. The implementation of sp0 was correct >> as far as I can tell, but sp0 was much weirder on x86_32 than I >> realized. It has the following issues: >> >> - Init's sp0 is inconsistent with everything else's: non-init tasks >> are offset by 8 bytes. (I have no idea why, and the comment is unhelpful.) >> >> - vm86 does crazy things to sp0. >> >> Fix it up by replacing this_cpu_sp0() with >> current_top_of_stack() and using a new percpu variable to track >> the top of the stack on x86_32. > > Looks like the hope that tss.sp0 is a reliable variable > which points to top of stack didn't really play out :( > > Recent relevant commits in x86/entry were: > > x86/asm/entry: Add this_cpu_sp0() to read sp0 for the current cpu > - added accessor to tss.sp0 > "We currently store references to the top of the kernel stack in > multiple places: kernel_stack (with an offset) and > init_tss.x86_tss.sp0 (no offset). The latter is defined by > hardware and is a clean canonical way to find the top of the > stack. Add an accessor so we can start using it." > > x86/asm/entry: Switch all C consumers of kernel_stack to this_cpu_sp0() > - equivalent change, no win/no loss > > x86/asm/entry/64/compat: Change the 32-bit sysenter code to use sp0 > - Even though it did remove one insn, we can get the same > if KERNEL_STACK_OFFSET will be eliminated > > x86: Delay loading sp0 slightly on task switch > - simple fix, nothing needed to be added > > x86: Replace this_cpu_sp0 with current_top_of_stack and fix it on x86_32 > - added a percpu var cpu_current_top_of_stack > - needs to set it in do_boot_cpu() > - added ifdef forest: > +#ifdef CONFIG_X86_64 > return this_cpu_read_stable(cpu_tss.x86_tss.sp0); > +#else > + /* sp0 on x86_32 is special in and around vm86 mode. */ > + return this_cpu_read_stable(cpu_current_top_of_stack); > +#endif > > > > End result is, now 32-bit kernel has two per-cpu vartiables, > cpu_current_top_of_stack and kernel_stack. > > cpu_current_top_of_stack is essentially "real top of stack", > and kernel_stack is "real top of stack - KERNEL_STACK_OFFSET". > > When/if we get rid of KERNEL_STACK_OFFSET, > we can also get rid of kernel_stack, since it will be the same as > cpu_current_top_of_stack (which is a better name anyway). Exactly. I think the next step might be to decouple GET_THREAD_INFO and friends from kernel_stack. I think that might be enough to get rid of kernel_stack on 32-bit. 64 has two other remaining users: the syscall entries. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
|