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    IRQ: Maintain regs pointer globally rather than passing to IRQ handlers · 7d12e780
    David Howells authored
    Maintain a per-CPU global "struct pt_regs *" variable which can be used instead
    of passing regs around manually through all ~1800 interrupt handlers in the
    Linux kernel.
    The regs pointer is used in few places, but it potentially costs both stack
    space and code to pass it around.  On the FRV arch, removing the regs parameter
    from all the genirq function results in a 20% speed up of the IRQ exit path
    (ie: from leaving timer_interrupt() to leaving do_IRQ()).
    Where appropriate, an arch may override the generic storage facility and do
    something different with the variable.  On FRV, for instance, the address is
    maintained in GR28 at all times inside the kernel as part of general exception
    Having looked over the code, it appears that the parameter may be handed down
    through up to twenty or so layers of functions.  Consider a USB character
    device attached to a USB hub, attached to a USB controller that posts its
    interrupts through a cascaded auxiliary interrupt controller.  A character
    device driver may want to pass regs to the sysrq handler through the input
    layer which adds another few layers of parameter passing.
    I've build this code with allyesconfig for x86_64 and i386.  I've runtested the
    main part of the code on FRV and i386, though I can't test most of the drivers.
    I've also done partial conversion for powerpc and MIPS - these at least compile
    with minimal configurations.
    This will affect all archs.  Mostly the changes should be relatively easy.
    Take do_IRQ(), store the regs pointer at the beginning, saving the old one:
    	struct pt_regs *old_regs = set_irq_regs(regs);
    And put the old one back at the end:
    Don't pass regs through to generic_handle_irq() or __do_IRQ().
    In timer_interrupt(), this sort of change will be necessary:
    	-	update_process_times(user_mode(regs));
    	-	profile_tick(CPU_PROFILING, regs);
    	+	update_process_times(user_mode(get_irq_regs()));
    	+	profile_tick(CPU_PROFILING);
    I'd like to move update_process_times()'s use of get_irq_regs() into itself,
    except that i386, alone of the archs, uses something other than user_mode().
    Some notes on the interrupt handling in the drivers:
     (*) input_dev() is now gone entirely.  The regs pointer is no longer stored in
         the input_dev struct.
     (*) finish_unlinks() in drivers/usb/host/ohci-q.c needs checking.  It does
         something different depending on whether it's been supplied with a regs
         pointer or not.
     (*) Various IRQ handler function pointers have been moved to type
    Signed-Off-By: default avatarDavid Howells <>
    (cherry picked from 1b16e7ac850969f38b375e511e3fa2f474a33867 commit)