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Date:   Wed, 24 May 2023 14:39:50 -0700
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Subject: Re: [PATCH v7 08/14] KVM: Rename mmu_notifier_*
From:   Sean Christopherson <seanjc@google.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Kautuk Consul <kconsul@linux.vnet.ibm.com>,
        Chao Peng <chao.p.peng@linux.intel.com>, kvm@vger.kernel.org,
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        Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
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        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>, Mike Rapoport <rppt@kernel.org>,
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On Wed, May 24, 2023, Peter Zijlstra wrote:
> On Wed, May 24, 2023 at 01:16:03PM -0700, Sean Christopherson wrote:
> > 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().

Ah, I take it s390's handcoded assembly routines are just a paranoid equivalents
and not truly special?  "l" and "st" do sound quite generic...

  commit 7657e41a0bd16c9d8b3cefe8fd5d6ac3c25ae4bf
  Author: Heiko Carstens <hca@linux.ibm.com>
  Date:   Thu Feb 17 13:13:58 2011 +0100

    [S390] atomic: use inline asm
    
    Use inline assemblies for atomic_read/set(). This way there shouldn't
    be any questions or subtle volatile semantics left.

static inline int __atomic_read(const atomic_t *v)
{
	int c;

	asm volatile(
		"	l	%0,%1\n"
		: "=d" (c) : "R" (v->counter));
	return c;
}

static inline void __atomic_set(atomic_t *v, int i)
{
	asm volatile(
		"	st	%1,%0\n"
		: "=R" (v->counter) : "d" (i));
}