From: Sean Christopherson <seanjc@google.com>
To: Nicolas Saenz Julienne <nsaenz@amazon.com>
Cc: pbonzini@redhat.com, kvm@vger.kernel.org, nh-open-source@amazon.com
Subject: Re: [kvm-unit-tests PATCH] x86: Make set/clear_bit() atomic
Date: Fri, 14 Feb 2025 17:18:24 -0800 [thread overview]
Message-ID: <Z6_rYA-8SJm_aQlY@google.com> (raw)
In-Reply-To: <20250214173644.22895-1-nsaenz@amazon.com>
On Fri, Feb 14, 2025, Nicolas Saenz Julienne wrote:
> x86 is the only architecture that defines set/clear_bit() as non-atomic.
> This makes it incompatible with arch-agnostic code that might implicitly
> require atomicity. And it was observed to corrupt the 'online_cpus'
> bitmap, as non BSP CPUs perform RmWs on the bitmap concurrently during
> bring up. See:
>
> ap_start64()
> save_id()
> set_bit(apic_id(), online_cpus)
>
> Address this by making set/clear_bit() atomic.
OMG, this is arguaby worse than the per-CPU stack/data mess. *sigh*
I'll grab this, I'm putting together a pull request for a few things.
next prev parent reply other threads:[~2025-02-15 1:18 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-02-14 17:36 [kvm-unit-tests PATCH] x86: Make set/clear_bit() atomic Nicolas Saenz Julienne
2025-02-15 1:18 ` Sean Christopherson [this message]
2025-02-24 17:23 ` Sean Christopherson
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