[Lguest] [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

[Ingo Molnar]

* Alexander van Heukelum <heukelum at mailshack.com> wrote:

> Hi all,
> 
> An x86 processor handles an interrupt (from an external source, 
> software generated or due to an exception), depending on the 
> contents if the IDT. Normally the IDT contains mostly interrupt 
> gates. Linux points each interrupt gate to a unique function. Some 
> are specific to some task (handling traps, IPI's, ...), the others 
> are stubs that push the interrupt number to the stack and jump to 
> 'common_interrupt'.
> 
> This patch removes the need for the stubs.

hm, the cost would be this new code:

> +.p2align
> +ENTRY(maininterrupt)
>  	RING0_INT_FRAME
> -vector=0
> -.rept NR_VECTORS
> -	ALIGN
> - .if vector
> -	CFI_ADJUST_CFA_OFFSET -4
> - .endif
> -1:	pushl $~(vector)
> -	CFI_ADJUST_CFA_OFFSET 4
> +	push %eax
> +	push %eax
> +	mov %cs,%eax
> +	shr $3,%eax
> +	and $0xff,%eax
> +	not %eax
> +	mov %eax,4(%esp)
> +	pop %eax
>  	jmp common_interrupt

.. which we were able to avoid before. A couple of segment register 
accesses, shifts, etc to calculate the vector - each of which can be 
quite costly (especially the segment register access - this is a 
relatively rare instruction pattern).

I'm not unconvicable, but we need to be conservative here: could you 
try to measure the full before/after cost of IRQ entry, to the cycle 
level? I'm curious what the performance impact is.

Also, this makes life probably a bit harder for Xen, which assumes 
that the GDT of the guest OS is small-ish. (Jeremy Cc:-ed)

	Ingo