On 5 October 2018 at 17:08, Andy Lutomirski <l...@amacapital.net> wrote:
>
>
>> On Oct 5, 2018, at 7:14 AM, Peter Zijlstra <pet...@infradead.org> wrote:
>>
>>> On Fri, Oct 05, 2018 at 10:13:25AM +0200, Ard Biesheuvel wrote:
>>> diff --git a/include/linux/ffp.h b/include/linux/ffp.h
>>> new file mode 100644
>>> index 000000000000..8fc3b4c9b38f
>>> --- /dev/null
>>> +++ b/include/linux/ffp.h
>>> @@ -0,0 +1,43 @@
>>> +/* SPDX-License-Identifier: GPL-2.0 */
>>> +
>>> +#ifndef __LINUX_FFP_H
>>> +#define __LINUX_FFP_H
>>> +
>>> +#include <linux/types.h>
>>> +#include <linux/compiler.h>
>>> +
>>> +#ifdef CONFIG_HAVE_ARCH_FFP
>>> +#include <asm/ffp.h>
>>> +#else
>>> +
>>> +struct ffp {
>>> + void (**fn)(void);
>>> + void (*default_fn)(void);
>>> +};
>>> +
>>> +#define DECLARE_FFP(_fn, _def) \
>>> + extern typeof(_def) *_fn; \
>>> + extern struct ffp const __ffp_ ## _fn
>>> +
>>> +#define DEFINE_FFP(_fn, _def) \
>>> + typeof(_def) *_fn = &_def; \
>>> + struct ffp const __ffp_ ## _fn \
>>> + = { (void(**)(void))&_fn, (void(*)(void))&_def }; \
>>> + EXPORT_SYMBOL(__ffp_ ## _fn)
>>> +
>>> +static inline void ffp_set_target(const struct ffp *m, void *new_fn)
>>> +{
>>> + WRITE_ONCE(*m->fn, new_fn);
>>> +}
>>> +
>>> +static inline void ffp_reset_target(const struct ffp *m)
>>> +{
>>> + WRITE_ONCE(*m->fn, m->default_fn);
>>> +}
>>> +
>>> +#endif
>>> +
>>> +#define SET_FFP(_fn, _new) ffp_set_target(&__ffp_ ## _fn, _new)
>>> +#define RESET_FFP(_fn) ffp_reset_target(&__ffp_ ## _fn)
>>> +
>>> +#endif
>>
>> I don't understand this interface. There is no wrapper for the call
>> site, so how are we going to patch all call-sites when you update the
>> target?
>
> I’m also confused.
>
> Anyway, we have patchable functions on x86. They’re called PVOPs, and they’re
> way overcomplicated.
>
> I’ve proposed a better way that should generate better code, be more
> portable, and be more maintainable. It goes like this.
>
> To call the function, you literally just call the default implementation.
> It *might* be necessary to call a nonexistent wrapper to avoid annoying
> optimizations. At build time, the kernel is built with relocations, so the
> object files contain relocation entries for the call. We collect these
> entries into a table. If we’re using the “nonexistent wrapper” approach, we
> can link in a .S or linker script to alias them to the default implementation.
>
> To patch them, we just patch them. It can’t necessarily be done concurrently
> because nothing forces the right alignment. But we can do it at boot time and
> module load time. (Maybe we can patch at runtime on architectures with
> appropriate instruction alignment. Or we ask gcc for an extension to align
> calls to a function.)
>
> Most of the machinery already exists: this is roughly how the module loader
> resolves calls outside of a module.
Yeah nothing is ever simple on x86 :-(
So are you saying the approach i use in patch #2 (which would
translate to emitting a jmpq instruction pointing to the default
implementation, and patching it at runtime to point elsewhere) would
not fly on x86?
