From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pf1-f202.google.com (mail-pf1-f202.google.com [209.85.210.202]) (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 48DDD194C96 for ; Tue, 21 Apr 2026 21:02:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.202 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776805377; cv=none; b=lbHE67HdpoS89RbDErkhJZFQOnKVi5vEntCBUftZyxT9du3uE5sfk4kejUhYfO/E6RJ1bkkDSUDc+Pe7hH04fKJiKuDpwZnz0iSj3zC/azbmf5kcxL3pwCTQRDETKaKj7pvNaooKYWTx8n930H42Xdh6wczDTJwU+S10+hNDrmA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776805377; c=relaxed/simple; bh=uj/Ek/OIU5KyWXwpRCvmuocnWKlUHMxQcaw3LAX4Q5Q=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=hCCRgzQya1uQ+UmzHGlsKh91iF3P9k5+eN2aXnLEfXTIkUJgMJaxP63aPZTHAG5JXN+EkuPoz7yg7c/db20ygmokGBywv4XUcQywVQBkE2Wk+dFebEFOnMYPHSLkxI4Mtpm2vhyBpbBW4BXc97MQc4jvX1AgBKGcgL8Z5aQKCLk= 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=XChayiNg; arc=none smtp.client-ip=209.85.210.202 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="XChayiNg" Received: by mail-pf1-f202.google.com with SMTP id d2e1a72fcca58-82fa2165c3eso2526542b3a.0 for ; Tue, 21 Apr 2026 14:02:56 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1776805376; x=1777410176; 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=h2A6E2FR5vQkEE+JGi0ud3Ckpr+NANcoJp/1DbuTKLw=; b=XChayiNgW3J7FgN0Fs+LoOJCpGTkdOxoH5ZQvx/KDU6gNEVxk2USB5GzWOSuIy1DRa YRqAVJPcl3GGtsX3imkTQJA3UcRqQgTJdoC9O35zwUbq7ZlKyPd6PvIJX71H1Rt3y3S1 O/3MINMoaqkLPT/Hf3pXj7FVcX4e6ds/8ePL6lozgVbYApAwgdTeubo57PSDFQYRHJvG R7zDThs2G23cMRQvWznty66T8W1krqthcWm89eA35tiuAl7kRObQtDFmEAnnZDqtFloO MNOroGZd2Q3doP0Bg7zfN8gXiXB+3dzVh3oTw3ARVZHaV2AophAX+criT9BIPokA27nN 9tXw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1776805376; x=1777410176; 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=h2A6E2FR5vQkEE+JGi0ud3Ckpr+NANcoJp/1DbuTKLw=; b=NbfAdFXoZNSrI58wpMxaVKhL3JjyF++5GBELIP+etU3fv5UBZXyxDXBtg1A18IvFlz NFcBVOnxKF4efyXLGeamjEfrpTHjpSdNvQNpoYtSwq+gkMEOuCsXYExELwPdMatXSXlE wIUvBvMH6lb6C6W5EPtgUnOZoBS8kozR1nUqjVt/HfM+MyYShHWn7Xb5bpSGRYEkerc8 H/6R51bQ2/TSUPGIhrvdiBCWuy/c8Epm8qqmPZLu/ZmwBmffKpPLWsFgdGJ/a1++kEff FtNFdFvDQVkgxzyst0c3Pd0Xu3CLJAJido7YUTrqNwfIeh/kP5XUmZ4jhnHqdtSVZJoz ZKhA== X-Forwarded-Encrypted: i=1; AFNElJ+QsAvtjuvJg1JVgeK1rdgk5fMdS7kZU8PPs+RE/DCYYjHCB7o0BDCiVwqaHpCwX9RqTc8=@vger.kernel.org X-Gm-Message-State: AOJu0YxUoVqT36BZWZl8jDCE7NOe/zTf46dLM9HepomrA0SvXSPyj1gT eB3DENC2j7yVxGfkkL1AMphn8OP8dPkQ0Dlf/+UDQbbCUL9VdM4axKjKSazZ9u7/dEQO5uKP67Y t+csRsQ== X-Received: from pfbk21.prod.google.com ([2002:a05:6a00:b015:b0:82f:790c:3922]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a00:b87:b0:827:441a:c970 with SMTP id d2e1a72fcca58-82f8c87e33amr19111815b3a.6.1776805375320; Tue, 21 Apr 2026 14:02:55 -0700 (PDT) Date: Tue, 21 Apr 2026 21:02:53 +0000 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <87eck8daot.ffs@tglx> <20260421111858.GH3126523@noisy.programming.kicks-ass.net> <20260421113212.GI3126523@noisy.programming.kicks-ass.net> <20260421113407.GE3102924@noisy.programming.kicks-ass.net> <20260421114940.GJ3126523@noisy.programming.kicks-ass.net> <87cxzsb5n0.ffs@tglx> <878qagb20x.ffs@tglx> Message-ID: Subject: Re: CPU Lockups in KVM with deferred hrtimer rearming From: Sean Christopherson To: Paolo Bonzini Cc: Thomas Gleixner , Jim Mattson , Peter Zijlstra , Binbin Wu , Vishal L Verma , kvm , Rick P Edgecombe , Binbin Wu , "the arch/x86 maintainers" , Paolo Bonzini Content-Type: text/plain; charset="us-ascii" On Tue, Apr 21, 2026, Paolo Bonzini wrote: > Il mar 21 apr 2026, 19:55 Sean Christopherson ha scritto: > > > > > > FWIW, VMX should work just like SVM if we clear VM_EXIT_ACK_INTR_ON_EXIT. > > > > Hell no. > > > > > I know. What's the point of that VM_EXIT_ACK_INTR_ON_EXIT exercise? Is > > > there any performance benefit or is it just used because it's there? > > > > There are performance benefits, and it preserves ordering [...] > > NMIs are unconditionally "acked" on VM-Exit. > > Not that I disagree but... > > > Even if performance is "fine", changing decades of fundamental KVM behavior is > > terrifying. > > ... it's not decades, ack on VM exit is actually relatively recent (10 > years out 20 :)). The reason why it was introduced is another killer > for the idea, though. Posted interrupts require it, Oh, I forgot about posted interrupts. So yeah, what Paolo said :-) > for some reason only known to Intel. My guess is because the ucode that morphs the notification vector into posted interrupt processing is a sub-clause of the ACK flow. And from an architectural perspective, having the CPU ACK/dismiss some IRQs (notification vectors) but not others (everything else) would be kludgy (even more so than special casing the notification vector already is).