Re: [PATCH 2/4] x86: get rid of KERNEL_STACK_OFFSET

[Ingo Molnar <mingo@kernel.org>]

* Denys Vlasenko <dvlasenk@redhat.com> 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.

Well, the original idea of percpu::kernel_stack was that of 
an optimization of the 64-bit system_call() path: to set up 
RSP as it has to be before we call into system calls.

This optimization has bitrotted away: because these days 
the first SAVE_ARGS in the 64-bit entry path modifies RSP 
as well, undoing the optimization.

But the fix should be to not touch RSP in SAVE_ARGS, to 
keep percpu::kernel_stack as an optimized entry point - 
with KERNEL_STACK_OFFSET pointing to.

So NAK - this should be fixed for real.

> 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.

Lets keep it in mind that in any case the micro-costs of 
the 32-bit entry path are almost always irrelevant: we 
optimize the 64-bit entry path, if that helps the 32-bit 
side as well then that's a happy coincidence, nothing more. 

If the 32-bit entry path can be optimized without affecting 
the 64-bit path then that's good, but we don't ever hurt 
the 64-bit path to make things easier or simpler for the 
32-bit path.

Thanks,

	Ingo