linux-kernel - Re: [PATCH v7 08/14] KVM: Rename mmu_notifier

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Message-ID: <20230524203336.GC3447678@hirez.programming.kicks-ass.net>
Date:   Wed, 24 May 2023 22:33:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Kautuk Consul <kconsul@...ux.vnet.ibm.com>,
        Chao Peng <chao.p.peng@...ux.intel.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-doc@...r.kernel.org, qemu-devel@...gnu.org,
        linux-kselftest@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v7 08/14] KVM: Rename mmu_notifier_*

On Wed, May 24, 2023 at 01:16:03PM -0700, Sean Christopherson wrote:

> Atomics aren't memory barriers on all architectures, e.g. see the various
> definitions of smp_mb__after_atomic().
> 
> Even if atomic operations did provide barriers, using an atomic would be overkill
> and a net negative.  On strongly ordered architectures like x86, memory barriers are
> just compiler barriers, whereas atomics may be more expensive. 

Not quite, smp_{r,w}mb() and smp_mb__{before,after}_atomic() are
compiler barriers on the TSO archs, but smp_mb() very much isn't. TSO
still allows stores to be delayed vs later loads (iow it doesn't pretend
to hide the store buffer).

> Of course, the only
> accesses outside of mmu_lock are reads, so on x86 that "atomic" access is just a
> READ_ONCE() load, but that's not the case for all architectures.

This is true on *all* archs. atomic_set() and atomic_read() are no more
and no less than WRITE_ONCE() / READ_ONCE().

> Anyways, the point is that atomics and memory barriers are different things that
> serve different purposes.

This is true; esp. on the weakly ordered architectures where atomics do
not naturally imply any ordering.