Messages in this thread | | | Date | Wed, 25 Feb 2015 09:53:51 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 2/4] x86: get rid of KERNEL_STACK_OFFSET |
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* 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
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