On Tue, 24 Feb 2015 19:51:33 +0100 Denys Vlasenko <[email protected]> wrote:
> PER_CPU_VAR(kernel_stack) was set up in a way where it points > five stack slots below the top of stack. > > Presumably, it was done to avoid one "sub $5*8,%rsp" > in syscall/sysenter code paths, where iret frame needs to be > created by hand. > > Ironically, none of them benefit from this optimization, > since all of them need to allocate additional data on stack > (struct pt_regs), so they still have to perform subtraction. > And ia32_sysenter_target even needs to *undo* this optimization: > it constructs iret stack with pushes instead of movs, > so it needs to start right at the top. > > This patch eliminates KERNEL_STACK_OFFSET. > PER_CPU_VAR(kernel_stack) now points directly to top of stack. > pt_regs allocations are adjusted to allocate iret frame as well. > I always thought the KERNEL_STACK_OFFSET wasn't an optimization, but a buffer from the real top of stack, in case we had any off by one bugs, it wouldn't crash the system. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
