From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out28-149.mail.aliyun.com (out28-149.mail.aliyun.com [115.124.28.149]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2D40218E20; Sat, 13 Sep 2025 05:06:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.28.149 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757740011; cv=none; b=rqEcMbmEET+UZbPaI69apw4Zy5yIFgteYR2+6A5XhA1p/nrG5Fx8yyDnZg68J7kgTN29ogpR33OrYqRTsLQl0cmIcMdWLtONtcRb4zVdoDcjZ3KZhG3/OWKKW7G8E65KARCZu5N8O+d7cJhyP9zUjMRU1g5gUWUIWhRRB9x+sDU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757740011; c=relaxed/simple; bh=Z3Bsk4yeuSaWiOfI2QPSAKBu+5jfsIQ/9hW08Kg+oas=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=PpOvhSCpVfI7XfujAw35DWB0jw8IjddEp/iVYiKI4Lx11IF4h6ft8kim03Whrh130/ngakVo8YKBf9hGDHdTmm0N6x6pmdYxzXZi1qLc1IGBMWWboPbE9cW21IOR+vEe2QduUq0U0EnJR+jgWdSK4pokZuYyjvWnv0cECLLScb0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=antgroup.com; spf=pass smtp.mailfrom=antgroup.com; dkim=pass (1024-bit key) header.d=antgroup.com header.i=@antgroup.com header.b=DfFHlaZt; arc=none smtp.client-ip=115.124.28.149 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=antgroup.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=antgroup.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=antgroup.com header.i=@antgroup.com header.b="DfFHlaZt" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=antgroup.com; s=default; t=1757740004; h=Date:From:To:Subject:Message-ID:MIME-Version:Content-Type; bh=r1khi3w4XkNENikcoeGAA5mHsJaRYmGZ2SI2OOjmJew=; b=DfFHlaZtiR1yCRRQCD9IbEHH1VvxMt0Fjux6W4JZEaZOcZF7pLsuKjgPLWZ2fhkkjXU0nS1DJmqzERNexy5HO6MxCjtJAqurRyu98htIME3p5oLy9ONPJmxQz2Uyb28Vdy5GcGhqJC6gI/FeVEGukBDk1jDrW8JjSo3QAGtVJzk= Received: from localhost(mailfrom:houwenlong.hwl@antgroup.com fp:SMTPD_---.ee6pIz1_1757740003 cluster:ay29) by smtp.aliyun-inc.com; Sat, 13 Sep 2025 13:06:44 +0800 Date: Sat, 13 Sep 2025 13:06:43 +0800 From: Hou Wenlong To: Sean Christopherson Cc: Chao Gao , kvm@vger.kernel.org, Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , linux-kernel@vger.kernel.org Subject: Re: [PATCH] KVM: x86: Remove outdated comments and code in kvm_on_user_return() Message-ID: <20250913050643.GA50691@k08j02272.eu95sqa> References: <20250912093822.GA10794@k08j02272.eu95sqa> <20250912141132.GA85606@k08j02272.eu95sqa> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) On Fri, Sep 12, 2025 at 07:40:43AM -0700, Sean Christopherson wrote: > On Fri, Sep 12, 2025, Hou Wenlong wrote: > > On Fri, Sep 12, 2025 at 05:38:22PM +0800, Hou Wenlong wrote: > > > On Fri, Sep 12, 2025 at 04:35:00PM +0800, Chao Gao wrote: > > > > On Fri, Sep 12, 2025 at 03:35:29PM +0800, Hou Wenlong wrote: > > > > >The commit a377ac1cd9d7b ("x86/entry: Move user return notifier out of > > > > >loop") moved fire_user_return_notifiers() into the section with > > > > >interrupts disabled, so the callback kvm_on_user_return() cannot be > > > > >interrupted by kvm_arch_disable_virtualization_cpu() now. Therefore, > > > > >remove the outdated comments and local_irq_save()/local_irq_restore() > > > > >code in kvm_on_user_return(). > > > > > > > > > >Signed-off-by: Hou Wenlong > > > > >--- > > > > > arch/x86/kvm/x86.c | 16 +++++----------- > > > > > 1 file changed, 5 insertions(+), 11 deletions(-) > > > > > > > > > >diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > > > >index 33fba801b205..10afbacb1851 100644 > > > > >--- a/arch/x86/kvm/x86.c > > > > >+++ b/arch/x86/kvm/x86.c > > > > >@@ -568,18 +568,12 @@ static void kvm_on_user_return(struct user_return_notifier *urn) > > > > > struct kvm_user_return_msrs *msrs > > > > > = container_of(urn, struct kvm_user_return_msrs, urn); > > > > > struct kvm_user_return_msr_values *values; > > > > >- unsigned long flags; > > > > > > > > > >- /* > > > > >- * Disabling irqs at this point since the following code could be > > > > >- * interrupted and executed through kvm_arch_disable_virtualization_cpu() > > > > >- */ > > > > >- local_irq_save(flags); > > > > >- if (msrs->registered) { > > > > >- msrs->registered = false; > > > > >- user_return_notifier_unregister(urn); > > > > >- } > > > > >- local_irq_restore(flags); > > > > >+ lockdep_assert_irqs_disabled(); > > > > > > > > kvm_offline_cpu() may call into this function. But I am not sure if interrupts > > > > are disabled in that path. > > > > > > > Thanks for pointing that out. I see that interrupts are enabled in the > > > callback during the CPU offline test. I'll remove the > > > lockdep_assert_irqs_disabled() here. > > > > > > > Upon a second look, can we just disable interrupts in kvm_cpu_offline()? > > The other paths that call kvm_disable_virtualization_cpu() are all in an > > interrupt-disabled state, although it seems that > > kvm_disable_virtualization_cpu() cannot be reentered. > > Why do we care? I.e. what is the motivation for changing this code? I'm hesitant > to touch this code without good reason given its fragility and subtlety. Hi, Sean. I'm just reworking the shared MSRs part in our inner multi-KVM. First, I noticed that the comment mentions that kvm_on_user_return() can be interrupted or reentered, which is a little confusing to me. Then, I found that the comment is outdated, so I decided to remove it and also make changes to the code. I agree that this code is fragile, maybe just change the comment? Thanks!