Re: [PATCH v5 00/10] x86/asm: Compile-time asm code validation

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Wed, Jun 10, 2015 at 11:40:06AM -0700, Andi Kleen wrote:
> Josh Poimboeuf <jpoimboe@redhat.com> writes:
> 
> > On Wed, Jun 10, 2015 at 05:04:12PM +0200, Andi Kleen wrote:
> >> > > > - duplicate the destination code inside the function
> >> > > > - convert the jump to a call
> >> > > 
> >> > > That all won't work for a lot of cases.
> >> > 
> >> > Hm, could you give an example?
> >> 
> >> Just a standard *_user exception handler.
> >
> > I'm afraid I don't follow.  Exception handlers don't work via jump
> > instructions, but rather via CPU exceptions.
> >
> > Or are you talking about something else?
> 
> Let's take an example:
> 
> 102:
>         .section .fixup,"ax"
>         103:    addl %ecx,%edx                  /* ecx is zerorest also */
>         jmp copy_user_handle_tail
>        .previous
> 
>         _ASM_EXTABLE(100b,103b)
>         _ASM_EXTABLE(101b,103b)
>                 
> The exception handling code is part of the function, but it's out of line.

The jump instruction is in the .fixup section, not in the callable
function itself.  So it doesn't violate the asmvalidate rules.

> > Are you suggesting that we implement this gcc optimization in kernel asm
> > code?
> 
> It was how Linux traditionally implemented locking code for example.
> Have the hot path handle the uncontended fast path, and the slow path
> call.
> 
> I don't know if there is much left of it (a lot of it was removed because
> it was hard to describe in dwarf3, needs dwarf4). But it seems bad
> to completely disallow it.
> 
> But yes eventually gcc generated code should use it again, because it's
> great for icache usage if you measure it correctly at run time
> (not the broken "size" approach that is unfortunately far too common)

This patch set has no relationship to gcc generated code whatsoever.  So
it doesn't disallow anything there.

For kernel asm code, AFAIK, such a mechanism for hot/cold path
separation in separate sections doesn't exist today.  So it's not
"disallowed" there either.  It's just apparently not currently done.

If somebody were to create such a mechanism, I think we could
standardize it in such a way that it could be compatible with
asmvalidate.

-- 
Josh