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

Josh Poimboeuf
2015-06-10T15:32:09Z

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?

> > Well, I don't see how that's really a logical conclusion.
>
> What's special about assembler code?

Ok, I'll bite:

- it's human-generated
- it's much simpler
- it doesn't have stack metadata by default
- it has far fewer constraints

But I don't see this line of discussion going anywhere without any real
examples of why you need external jumps in asm functions...

> > But we're
> > probably being too vague here... Do you have any examples where you
> > really need to jump outside of a callable function?
>
> It's not needed, but it's an optimization to optimize icache usage.
> It is optional (-freorder-blocks-and-partition)
>
> In this case gcc splits the function into two (hot and cold)
>
> It's actually a nice optimization and it would be sad from stopping
> the kernel from using it.

Sorry if I wasn't clear, I was trying to ask for examples in kernel asm
code.

Are you suggesting that we implement this gcc optimization in kernel asm
code?

--
Josh