Re: [RFC PATCH 1/3] static_call: Add static call infrastructure
From: Josh Poimboeuf
Date: Fri Nov 09 2018 - 09:55:28 EST
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@xxxxxxxxxx> 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