From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f74.google.com (mail-pj1-f74.google.com [209.85.216.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CCC79D515 for ; Sat, 15 Feb 2025 01:18:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.74 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739582308; cv=none; b=bM2rPlKDXoUUO5p9fFSZAgC40dAt5+ugT+tI9mkItcD2VXDbXkopdW/xtkuIYcuj/NVsc/joiKvEQux1l/XKle1V0uGIyd8nUwgl7PIccw7KflwKRgwHR7lVIhftaJpircLvl+x9vd2N8PbOrI/KaC0BZL83eqPFLrcXTLOeaUo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739582308; c=relaxed/simple; bh=qKDfbSuXufzEEBqCtHrOLMr264SLSFXq5e7mh+uT6KQ=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=nq/gTgz4fLsFqt5f2PWcLI8yseSZtjLdptWmnHXvqri9DlUBYXuJh1c5PiqSxDkZ/afjxVX0bP80EHDdcXzpPLZjwqZtxIg1Wx3RNst2Jyhi9OTVgjOb5AyD0LAh49P8EDT/EaHN++Jb6rjGDsWA4CBigXTIbN5WUgDkjRsZezY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=zhWGplgr; arc=none smtp.client-ip=209.85.216.74 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="zhWGplgr" Received: by mail-pj1-f74.google.com with SMTP id 98e67ed59e1d1-2fc404aaed5so1000545a91.3 for ; Fri, 14 Feb 2025 17:18:26 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1739582306; x=1740187106; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=IhETc31pPMX6d10pDaCuaihkIgWNidfFyk/wqrnCFkc=; b=zhWGplgraTjf7SWdAY021f6LvVin9DE0vQXRX0GiFYQSOsDrvGjjaBgd/WbNZ6OOrh x5pycQUeFgW30zHVJeCdPMMIK6EedtzjPhna554fc8uM5QdP0rdt0RZkKc7eGbcIETOE zeXhV8uS66ZZm/dV8iHwXGD4kqoy2xeM8ABImIi+6vc3wRTDPvihIbD1nPinxsmYQ/0a Tfv7Z0YD0QvVXLbtIONHUn5bFSOHejrvmwuOkMy/JV/r/3lhPjXYDpYs/Ft0qdTyd1y5 dbYsystKIfnzT/zwcwmpZmkvVpmllmq6JUinDO2RWou24ZSFU2B78fVZETrm5diWl5+C StAw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1739582306; x=1740187106; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=IhETc31pPMX6d10pDaCuaihkIgWNidfFyk/wqrnCFkc=; b=eu9x91ftrkI+knfJw/OpO9N+2Jk5pb4Sc90PcIMrQo48sGVVNY9/sUdCGnife5N7bA lXnRRkK0+NcnEmU5Cf9GZChyTANOoBXUulMg4GNSlinqyTMcceQ5DAfEg2aL5XI112VF ACkGV2tA2mroBtl65/e6Juf6m/BHtCy/2FGN4/wwejKvCWO+Sv2hJgwsStD7Z5G1BfGO srzxWRisqPG6hunyuU25lcbjHwTXi25DqLZzaXe1m61VLMYcyFmRPo2AWLKuPRv7HbxJ D5fqXoSGCGD1dArOqE+qI9lRA2Mx6/pgt+OmSv+VV/xX7EWxHJvQ9IIOTYK781Ewo7uX wOpg== X-Forwarded-Encrypted: i=1; AJvYcCVyc0ng42wX5Ar/pAA6EXYdC3joKkmFXzI+6thGsIYuL2Zs2V0CZWGMXRO3JdhcYfX3gQg=@vger.kernel.org X-Gm-Message-State: AOJu0Ywo9sz3fq8PIxstYEfwcpkVTKATuUO3eW731yCsXdtcONOWm8qi 1fS7wXsl4WlWfCAfIYW9O+38GFCmwzkmdlVtCuSUX6frGr+BIaY53MqnMPGLswwowPyk7y8ZoTr Cwg== X-Google-Smtp-Source: AGHT+IFYN+5IYhfq59TLMoAewKhswKqafONG958OepP5wvMi/15AIeQXg0VyJE/PBX0Ee6n82j6Z2G2ixpQ= X-Received: from pfih19.prod.google.com ([2002:a05:6a00:2193:b0:730:7a4d:b849]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a20:258b:b0:1ee:7cc3:f7ae with SMTP id adf61e73a8af0-1ee8cbe5011mr2652716637.37.1739582306132; Fri, 14 Feb 2025 17:18:26 -0800 (PST) Date: Fri, 14 Feb 2025 17:18:24 -0800 In-Reply-To: <20250214173644.22895-1-nsaenz@amazon.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250214173644.22895-1-nsaenz@amazon.com> Message-ID: Subject: Re: [kvm-unit-tests PATCH] x86: Make set/clear_bit() atomic From: Sean Christopherson To: Nicolas Saenz Julienne Cc: pbonzini@redhat.com, kvm@vger.kernel.org, nh-open-source@amazon.com Content-Type: text/plain; charset="us-ascii" 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.