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

Ingo Molnar
2015-02-25T08:54:01Z

* 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