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SubjectRe: [PATCH v3 8/7] TESTING_ONLY x86/entry: reduce static footprint of idtentry

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, Feb 12, 2018 at 1:37 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > Ok, so this does not look _that_ complicated, and the .text savings are
> > significant:
>
> Honestly, I think we should do it. 3kB of assembly code is noticeable.
>
> Also, that patch actually allows more cleanups and simplifications.
> Look at the "interrupt" macro, which is used by 'apicinterrupt3', and
> has a number of uses that way.
>
> That code could be unified a lot, right now it does:
>
> testb $3, CS-ORIG_RAX(%rsp)
> jz 1f
> SWAPGS
> call switch_to_thread_stack
> 1:
>
> ALLOC_PT_GPREGS_ON_STACK
> SAVE_C_REGS
> SAVE_EXTRA_REGS
> ENCODE_FRAME_POINTER
>
> testb $3, CS(%rsp)
> jz 1f
>
> /*
> * IRQ from user mode.
> *
> * We need to tell lockdep that IRQs are off. We can't do this until
> * we fix gsbase, and we should do it before enter_from_user_mode
> * (which can take locks). Since TRACE_IRQS_OFF idempotent,
> * the simplest way to handle it is to just call it twice if
> * we enter from user mode. There's no reason to optimize this since
> * TRACE_IRQS_OFF is a no-op if lockdep is off.
> */
> TRACE_IRQS_OFF
>
> CALL_enter_from_user_mode
>
> 1:
> ENTER_IRQ_STACK old_rsp=%rdi
> /* We entered an interrupt context - irqs are off: */
> TRACE_IRQS_OFF
>
> and *all* of that could be in a helper function rather than be
> duplicated. and the apicinterrupt3 macro should end up just expanding
> to
>
> callq helper
> pushq $~(\num)
> callq \fn
> jmp ret_from_intr
>
> instead of expanding to all that code.
>
> But that would require that same "save_ret" logic.
>
> So it's not just the idtentry cases that can use this trick.
>
> I admit that the trick isn't pretty, but it's not *horribly* ugly either.

Ok, agreed!

Dominik, could you please do this on top latest tip:master? I have applied all the
other patches. Note that patch #8 now conflicts with recent annotation fixes.

Thanks,

Ingo

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