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

Denys Vlasenko
2015-02-25T12:48:46Z

On Wed, Feb 25, 2015 at 9:53 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * 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.

Yes, I figured this is how it was supposed to work.

> 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.

IOW, the proposal is to set KERNEL_STACK_OFFSET
to SIZEOF_PTREGS. I can do that.

However.

There is an ortogonal idea we were discussing: to save
registers and construct iret frame using PUSH insns, not MOVs.
IIRC Andy and Linus liked it. I am ambivalent: the code will be smaller,
but might get slower (at least on some CPUs).
If we go that way, we will require KERNEL_STACK_OFFSET = 0
(IOW: the current patch).

The decision on how exactly we should fix KERNEL_STACK_OFFSET
(set it to SIZEOF_PTREGS or to zero) depends on whether
we switch to using PUSHes, or not. What do you think?

--
vda