Re: [PATCH 0/5] x86: Cleanups around slow_down_io()

[Jürgen Groß]

On 14.12.25 09:05, Ingo Molnar wrote:
> * Juergen Gross <jgross@xxxxxxxx> wrote:
>
> > While looking at paravirt cleanups I stumbled over slow_down_io() and
> > the related REALLY_SLOW_IO define.
> >
> > Especially REALLY_SLOW_IO is a mess, which is proven by 2 completely
> > wrong use cases.
> >
> > Do several cleanups, resulting in a deletion of REALLY_SLOW_IO and the
> > io_delay() paravirt function hook.
> >
> > Patches 2 and 3 are not changing any functionality, but maybe they
> > should? As the potential bug has been present for more than a decade
> > now, I went with just deleting the useless "#define REALLY_SLOW_IO".
> > The alternative would be to do something similar as in patch 5.
> >
> > Juergen Gross (5):
> >    x86/paravirt: Replace io_delay() hook with a bool
> >    hwmon/lm78: Drop REALLY_SLOW_IO setting
> >    hwmon/w83781d: Drop REALLY_SLOW_IO setting
> >    block/floppy: Don't use REALLY_SLOW_IO for delays
> >    x86/io: Remove REALLY_SLOW_IO handling
> >
> >   arch/x86/include/asm/floppy.h         | 27 ++++++++++++++++++++++-----
> >   arch/x86/include/asm/io.h             | 12 +++++-------
> >   arch/x86/include/asm/paravirt.h       | 11 +----------
> >   arch/x86/include/asm/paravirt_types.h |  3 +--
> >   arch/x86/kernel/cpu/vmware.c          |  2 +-
> >   arch/x86/kernel/kvm.c                 |  8 +-------
> >   arch/x86/kernel/paravirt.c            |  3 +--
> >   arch/x86/xen/enlighten_pv.c           |  6 +-----
> >   drivers/block/floppy.c                |  2 --
> >   drivers/hwmon/lm78.c                  |  5 +++--
> >   drivers/hwmon/w83781d.c               |  5 +++--
> >   11 files changed, 39 insertions(+), 45 deletions(-)
>
> I think we should get rid of *all* io_delay hacks, they might have been
> relevant in the days of i386 systems, but we don't even support i386
> CPUs anymore. Should it cause any regressions, it's easy to bisect to.
> There's been enough changes around all these facilities that the
> original timings are probably way off already, so we've just been
> cargo-cult porting these to newer kernels essentially.

Fine with me.

Which path to removal of io_delay would you (and others) prefer?

1. Ripping it out immediately.

2. Hiding it behind a default-off config option for a few kernel versions
   before removing it.

3. Using CONFIG_IO_DELAY_NONE as the default io_delay_type before ripping it
   out.

4. Using CONFIG_IO_DELAY_NONE as the default io_delay_type before hiding it
   behind a default-off config option, then rip it out later.

In cases 2-4 I'd still like to have patch 1 of my series applied, as it will
make paravirt rework easier.


Juergen

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