Re: [PATCH RFC 0/3] Static calls

[Andy Lutomirski]

On Sun, Nov 11, 2018 at 9:02 PM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> > On Fri, Nov 09, 2018 at 08:28:11AM +0100, Ingo Molnar wrote:
> > > > - I'm not sure about the objtool approach.  Objtool is (currently)
> > > >   x86-64 only, which means we have to use the "unoptimized" version
> > > >   everywhere else.  I may experiment with a GCC plugin instead.
> > >
> > > I'd prefer the objtool approach. It's a pretty reliable first-principles
> > > approach while GCC plugin would have to be replicated for Clang and any
> > > other compilers, etc.
> >
> > The benefit of a plugin is that we'd only need two of them: GCC and
> > Clang.  And presumably, they'd share a lot of code.
> >
> > The prospect of porting objtool to all architectures is going to be much
> > more of a daunting task (though we are at least already considering it
> > for some arches).
>
> Which architectures would benefit from ORC support the most?
>
> I really think that hard reliance on GCC plugins is foolish - but maybe
> Clang's plugin infrastructure is a guarantee that it remains a sane and
> usable interface.
>
> > > I'd be very happy with a demonstrated paravirt optimization already -
> > > i.e. seeing the before/after effect on the vmlinux with an x86 distro
> > > config.
> > >
> > > All major Linux distributions enable CONFIG_PARAVIRT=y and
> > > CONFIG_PARAVIRT_XXL=y on x86 at the moment, so optimizing it away as much
> > > as possible in the 99.999% cases where it's not used is a primary
> > > concern.
> >
> > For paravirt, I was thinking of it as more of a cleanup than an
> > optimization.  The paravirt patching code already replaces indirect
> > branches with direct ones -- see paravirt_patch_default().
> >
> > Though it *would* reduce the instruction footprint a bit, as the 7-byte
> > indirect calls (later patched to 5-byte direct + 2-byte nop) would
> > instead be 5-byte direct calls to begin with.
>
> Yes.

It would be a huge cleanup IMO -- the existing PVOP call stuff is
really quite ugly IMO.  Also, the existing stuff tries to emulate the
semantics of passing parameters of unknown types using asm
constraints, and I just don't believe that GCC does what we want it to
do.  In general, passing the *value* of a pointer to asm doesn't seem
to convince gcc that the pointed-to value is used by the asm, and this
makes me nervous.  See commit 715bd9d12f84d8f5cc8ad21d888f9bc304a8eb0b
as an example of this.  In a similar vein, the existing PVOP calls
have a "memory" clobber, and that's not free.