Date:   Tue, 7 Mar 2023 21:43:30 +0100
From:   Jakub Jelinek <jakub@redhat.com>
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Stephane Eranian <eranian@google.com>,
        linux-kernel@vger.kernel.org, tony.luck@intel.com,
        reinette.chatre@intel.com, fenghua.yu@intel.com,
        peternewman@google.com, james.morse@arm.com, babu.moger@amd.com,
        ananth.narayan@amd.com, vschneid@redhat.com,
        Nathan Chancellor <nathan@kernel.org>,
        clang-built-linux <llvm@lists.linux.dev>,
        Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-toolchains@vger.kernel.org
Subject: Re: [PATCH] x86/resctrl: avoid compiler optimization in
 __resctrl_sched_in
Message-ID: <ZAeh8g0nr3IFRSVI@tucnak>
Reply-To: Jakub Jelinek <jakub@redhat.com>
References: <20230303231133.1486085-1-eranian@google.com>
 <20230306120106.GE1267364@hirez.programming.kicks-ass.net>
 <CAKwvOdnRvd5KK01awAyeyt5S36TPPW4_8Z6YL1r4gB-pBrHTbg@mail.gmail.com>
 <20230307113545.GB2017917@hirez.programming.kicks-ass.net>
 <20230307184315.GS25951@gate.crashing.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230307184315.GS25951@gate.crashing.org>
Precedence: bulk

On Tue, Mar 07, 2023 at 12:43:16PM -0600, Segher Boessenkool wrote:
> 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.

Are we actually talking here about "p" constraint or about p/P (x86) modifiers
(asm ("%p0" : : "i" (42));)?

	Jakub