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

[Cyrill Gorcunov]

[Alexander van Heukelum - Tue, Nov 04, 2008 at 01:28:39PM +0100]
| 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.
| 
| An interrupt gate contains a FAR pointer to the interrupt
| handler, meaning that the code segment of the interrupt
| handler is also reloaded. Instead of pointing each (non-
| specific) interrupt gate to a unique handler, we set a
| unique code segment and use a common handler. When the
| handler finishes the code segment is restored to the
| 'normal'/previous one.
| 
| In order to have a unique code segment for each interrupt
| vector, the GDT is extended to 512 entries (1 full page),
| and the last half of the page describes identical code
| segments (identical, except for the number in the cs
| register), which are refered to by the 256 interrupt
| gates.
| 
| In this version, even the specialized handlers get run
| with their code segment switched. This is not necessary,
| but I like the fact that in a register dump one can now
| see from the code segment that the code is ran due to
| a (hard) interrupt. The exception I made is the int 0x80
| (syscall), which runs with the normal kernel code segment.
| 
| 
| Concluding: changing interrupt handling to this way
| removes quite a bit of source code. It also removes the
| need for the interrupt stubs and, on i386, pointers to
| them. This saves a few kilobytes of code. The page
| reserved for the GDT is now fully used. The cs register
| indicating directly that code is executed on behalf of
| a (hardware) interrupt is a nice debugging aid. This way
| of handling interrupts also leads to cleaner code: this
| patch already gets rid of some 'ugly' macro magic in
| entry_32.S and irqinit_64.c.
| 
| More cleanup is certainly possible, but I have tried to
| keep the changes local and small. If switching code
| segments is too expensive for some paths, that can be
| fixed by not doing that ;).
| 
| I'ld welcome some numbers on a few benchmarks on real
| hardware (I only tested on qemu: debian runs without
| noticable differences before/after this patch).
| 
| Greetings,
|     Alexander
| 
| P.S. Just in case someone thinks this is a great idea and
| testing and benchmarking goes well...
|
...

Hi Alexander, great done!

not taking into account the cost of cs reading (which I
don't suspect to be that expensive apart from writting,
on the other hand I guess walking on GDT entries could
be not that cheap especially with new segments you propose,
I guess cpu internally check for segment to be the same
and do not reload it again even if it's described as FAR
pointer but I could be wrong so Andi CC'ed :)

A small nit in implementation:

entry_32.S:
+	push %eax
+	push %eax
+	mov %cs,%eax
+	shr $3,%eax
+	and $0xff,%eax
+	not %eax
+	mov %eax,4(%esp)
+	pop %eax

CFI_ADJUST_CFA_OFFSET missed?

		- Cyrill -