linux-kernel - Re: [PATCH] x86/resctrl: avoid compiler optimization in __resctrl_sched

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Date:   Tue, 7 Mar 2023 12:43:16 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephane Eranian <eranian@...gle.com>,
        linux-kernel@...r.kernel.org, tony.luck@...el.com,
        reinette.chatre@...el.com, fenghua.yu@...el.com,
        peternewman@...gle.com, james.morse@....com, babu.moger@....com,
        ananth.narayan@....com, vschneid@...hat.com,
        Nathan Chancellor <nathan@...nel.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-toolchains@...r.kernel.org
Subject: Re: [PATCH] x86/resctrl: avoid compiler optimization in __resctrl_sched_in

Hi!

On Tue, Mar 07, 2023 at 12:35:45PM +0100, Peter Zijlstra wrote:
> So per that summary, I'm going to nit-pick and state we very much want
> CSE. CSE good. What we don't want it violating store-load ordering.

So you need to describe exactly what you *do* want.  There is no way to
forbid most otherwise valid things.  But you can express pretty much all
dependencies.

> Oh, geez, what a twisty tale that... So Linus knew back in '09 that "p"
> was icky, but it sorta was the only thing and it 'worked' -- until now
> :/

The "p" constraint is just like any other address_constraint, in most
aspects.  Since this is very specific to "p", that limits what is going
on to really just one thing.

For "p", after reload, strict_memory_address_addr_space_p is used.  That
is, targetm.addr_space.legitimate_address_p with strict set to true.  On
x86 that limits what registers can be used?  So I guess that made things
accidentally work before?

> So ideally we'd get something that respects the whole store-load
> ordering but still allows agressive CSE. And works for both toolchains.
> Small ask, I know :-)

Well, what is the ordering you need respected, *exactly*?

Segher