[RFC please help] membarrier: Rewrite sync_core_before_usermode()

[Andy Lutomirski]

On Mon, Dec 28, 2020 at 4:11 PM Nicholas Piggin <npiggin at gmail.com> wrote:
>
> Excerpts from Andy Lutomirski's message of December 28, 2020 4:28 am:
> > The old sync_core_before_usermode() comments said that a non-icache-syncing
> > return-to-usermode instruction is x86-specific and that all other
> > architectures automatically notice cross-modified code on return to
> > userspace.  Based on my general understanding of how CPUs work and based on
> > my atttempt to read the ARM manual, this is not true at all.  In fact, x86
> > seems to be a bit of an anomaly in the other direction: x86's IRET is
> > unusually heavyweight for a return-to-usermode instruction.
>
> "sync_core_before_usermode" as I've said says nothing to arch, or to the
> scheduler, or to membarrier.

Agreed.  My patch tries to fix this.  I agree that the name is bad and
could be improved further.  We should define what
membarrier(...SYNC_CORE) actually does and have arch hooks to make it
happen.

> > So let's drop any pretense that we can have a generic way implementation
> > behind membarrier's SYNC_CORE flush and require all architectures that opt
> > in to supply their own.  This means x86, arm64, and powerpc for now.  Let's
> > also rename the function from sync_core_before_usermode() to
> > membarrier_sync_core_before_usermode() because the precise flushing details
> > may very well be specific to membarrier, and even the concept of
> > "sync_core" in the kernel is mostly an x86-ism.
>
> The concept of "sync_core" (x86: serializing instruction, powerpc: context
> synchronizing instruction, etc) is not an x86-ism at all. x86 just wanted
> to add a serializing instruction to generic code so it grew this nasty API,
> but the concept applies broadly.

I mean that the mapping from the name "sync_core" to its semantics is
x86 only.  The string "sync_core" appears in the kernel only in
arch/x86, membarrier code, membarrier docs, and a single SGI driver
that is x86-only.  Sure, the idea of serializing things is fairly
generic, but exactly what operations serialize what, when things need
serialization, etc is quite architecture specific.

Heck, on 486 you serialize the instruction stream with JMP.

> > +static inline void membarrier_sync_core_before_usermode(void)
> > +{
> > +     /*
> > +      * XXX: I know basically nothing about powerpc cache management.
> > +      * Is this correct?
> > +      */
> > +     isync();
>
> This is not about memory ordering or cache management, it's about
> pipeline management. Powerpc's return to user mode serializes the
> CPU (aka the hardware thread, _not_ the core; another wrongness of
> the name, but AFAIKS the HW thread is what is required for
> membarrier). So this is wrong, powerpc needs nothing here.

Fair enough.  I'm happy to defer to you on the powerpc details.  In
any case, this just illustrates that we need feedback from a person
who knows more about ARM64 than I do.