Re: [RFC PATCH 1/3] static_call: Add static call infrastructure

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, Nov 09, 2018 at 10:51:16AM +0100, Ard Biesheuvel wrote:
> Hi Josh,
> 
> Thanks a lot for looking into this.
> 
> On 8 November 2018 at 22:15, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > Add a static call infrastructure.  Static calls use code patching to
> > hard-code function pointers into direct branch instructions.  They give
> > the flexibility of function pointers, but with improved performance.
> > This is especially important for cases where retpolines would otherwise
> > be used, as retpolines can significantly impact performance.
> >
> > This code is heavily inspired by the jump label code (aka "static
> > jumps"), as some of the concepts are very similar.
> >
> > There are three implementations, depending on arch support:
> >
> > 1) optimized: patched call sites (CONFIG_HAVE_STATIC_CALL_OPTIMIZED)
> > 2) unoptimized: patched trampolines (CONFIG_HAVE_STATIC_CALL_UNOPTIMIZED)
> 
> Could we move to an idiom like inline/out of line? The unoptimized
> variant may be perfectly adequate for many arches, and 'unoptimized'
> sounds like there is something wrong with it.

Yeah, inline/out-of-line would be better.  It's both more descriptive
and less derogatory :-)

> > + * Usage example:
> > + *
> > + *   # Start with the following functions (with identical prototypes):
> > + *   int func_a(int arg1, int arg2);
> > + *   int func_b(int arg1, int arg2);
> > + *
> > + *   # Define a 'my_key' reference, associated with func_a() by default
> > + *   DEFINE_STATIC_CALL(my_key, func_a);
> > + *
> > + *   # Call func_a()
> > + *   static_call(my_key, arg1, arg2);
> > + *
> > + *   # Update 'my_key' to point to func_b()
> > + *   static_call_update(my_key, func_b);
> > + *
> > + *   # Call func_b()
> > + *   static_call(my_key, arg1, arg2);
> > + *
> 
> Any way we can revert to the default implementation? That would be
> useful for, e.g.,  unloading modules that provide an alternative
> implementation.

I saw your original implementation had a "reset to default", but from
what I can tell, most users wouldn't need that.

Assuming the caller knows what the original dest function was, can't
they just call static_call_update(my_key, orig_func) directly?

> > diff --git a/include/linux/static_call_types.h b/include/linux/static_call_types.h
> > new file mode 100644
> > index 000000000000..7dd4b3d7ec2b
> > --- /dev/null
> > +++ b/include/linux/static_call_types.h
> > @@ -0,0 +1,19 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _STATIC_CALL_TYPES_H
> > +#define _STATIC_CALL_TYPES_H
> > +
> > +#include <linux/stringify.h>
> > +
> > +#define STATIC_CALL_TRAMP_PREFIX ____static_call_tramp_
> > +#define STATIC_CALL_TRAMP_PREFIX_STR __stringify(STATIC_CALL_TRAMP_PREFIX)
> > +
> > +#define STATIC_CALL_TRAMP(key) STATIC_CALL_TRAMP_PREFIX##key
> > +#define STATIC_CALL_TRAMP_STR(key) __stringify(STATIC_CALL_TRAMP(key))
> > +
> 
> I needed to apply
> 
> diff --git a/include/linux/static_call_types.h
> b/include/linux/static_call_types.h
> index 7dd4b3d7ec2b..6859b208de6e 100644
> --- a/include/linux/static_call_types.h
> +++ b/include/linux/static_call_types.h
> @@ -7,7 +7,7 @@
>  #define STATIC_CALL_TRAMP_PREFIX ____static_call_tramp_
>  #define STATIC_CALL_TRAMP_PREFIX_STR __stringify(STATIC_CALL_TRAMP_PREFIX)
> 
> -#define STATIC_CALL_TRAMP(key) STATIC_CALL_TRAMP_PREFIX##key
> +#define STATIC_CALL_TRAMP(key) __PASTE(STATIC_CALL_TRAMP_PREFIX, key)
>  #define STATIC_CALL_TRAMP_STR(key) __stringify(STATIC_CALL_TRAMP(key))
> 
>  /* The static call site table is created by objtool. */
> 
> or I end up with
> 
> 0000000000000000 <STATIC_CALL_TRAMP_PREFIXfoo>:
>    0: 14000000 b a8 <foo_a>
>    4: d503201f nop

Ha, oops.  That was part of a last minute cleanup to share the macros
with objtool.


-- 
Josh