linux-kernel - Re: [PATCH v2 4/4] x86/percpu: Use C for percpu read/write accessors

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Message-ID: <CAFULd4b2vmJrUReQw1TXvndzB=DfmHOvNwM9=bU_4O86s4UO3g@mail.gmail.com>
Date:   Wed, 4 Oct 2023 22:22:12 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Nadav Amit <namit@...are.com>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v2 4/4] x86/percpu: Use C for percpu read/write accessors

On Wed, Oct 4, 2023 at 10:20 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Wed, 4 Oct 2023 at 13:12, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > On Wed, 4 Oct 2023 at 13:08, Uros Bizjak <ubizjak@...il.com> wrote:
> > >
> > > You get a store forwarding stall when you write a bigger operand to
> > > memory and then read part of it, if the smaller part doesn't start at
> > > the same address.
> >
> > I don't think that has been true for over a decade now.
> >
> > Afaik, any half-way modern Intel and AMD cores will forward any fully
> > contained load.
>
> https://www.agner.org/optimize/microarchitecture.pdf
>
> See for example pg 136 (Sandy Bridge / Ivy Bridge):
>
>  "Store forwarding works in the following cases:
>   ..
>   • When a write of 64 bits or less is followed by a read of a smaller
> size which is fully contained in the write address range, regardless
> of alignment"
>
> and for AMD Zen cores:
>
>   "Store forwarding of a write to a subsequent read works very well in
> all cases, including reads from a part of the written data"
>
> So forget the whole "same address" rule. It's simply not true or
> relevant any more.

No problem then, we will implement the optimization in the compiler.

Thanks,
Uros.