From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 771F3BA44 for ; Tue, 7 Mar 2023 17:50:27 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D7D30C4339E; Tue, 7 Mar 2023 17:50:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1678211427; bh=5PY/Pxx78wE4EwQtttQF8UnMsI02CIxsIwSq8m23V6M=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=V4hjbMJjEKSrL1UpDUk7wa/wgZLQKzOKJGOOEOUc8F62B6xF+kAri0UvGxVpKgVMy j9L/ATJQrGRvUydT56XLGfqXXKA9brUNmoF/ZW2EwfqHhTb6ZSBFKgRBSkYVCj2Eo1 CaJFO8eFx9MqcFvXLBb1O2+RH3GjzSEK7lrXY6jE= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Suravee Suthikulpanit , Maxim Levitsky , Sean Christopherson , Paolo Bonzini Subject: [PATCH 6.2 0827/1001] KVM: x86: Dont inhibit APICv/AVIC if xAPIC ID mismatch is due to 32-bit ID Date: Tue, 7 Mar 2023 17:59:59 +0100 Message-Id: <20230307170057.652868823@linuxfoundation.org> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230307170022.094103862@linuxfoundation.org> References: <20230307170022.094103862@linuxfoundation.org> User-Agent: quilt/0.67 Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit From: Sean Christopherson commit f651a008954803d7bb2d85b7042d0fd46133d782 upstream. Truncate the vcpu_id, a.k.a. x2APIC ID, to an 8-bit value when comparing it against the xAPIC ID to avoid false positives (sort of) on systems with >255 CPUs, i.e. with IDs that don't fit into a u8. The intent of APIC_ID_MODIFIED is to inhibit APICv/AVIC when the xAPIC is changed from it's original value, The mismatch isn't technically a false positive, as architecturally the xAPIC IDs do end up being aliased in this scenario, and neither APICv nor AVIC correctly handles IPI virtualization when there is aliasing. However, KVM already deliberately does not honor the aliasing behavior that results when an x2APIC ID gets truncated to an xAPIC ID. I.e. the resulting APICv/AVIC behavior is aligned with KVM's existing behavior when KVM's x2APIC hotplug hack is effectively enabled. If/when KVM provides a way to disable the hotplug hack, APICv/AVIC can piggyback whatever logic disables the optimized APIC map (which is what provides the hotplug hack), i.e. so that KVM's optimized map and APIC virtualization yield the same behavior. For now, fix the immediate problem of APIC virtualization being disabled for large VMs, which is a much more pressing issue than ensuring KVM honors architectural behavior for APIC ID aliasing. Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base") Reported-by: Suravee Suthikulpanit Cc: stable@vger.kernel.org Reviewed-by: Maxim Levitsky Signed-off-by: Sean Christopherson Message-Id: <20230106011306.85230-7-seanjc@google.com> Signed-off-by: Paolo Bonzini Signed-off-by: Greg Kroah-Hartman --- arch/x86/kvm/lapic.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) --- a/arch/x86/kvm/lapic.c +++ b/arch/x86/kvm/lapic.c @@ -2078,7 +2078,12 @@ static void kvm_lapic_xapic_id_updated(s if (KVM_BUG_ON(apic_x2apic_mode(apic), kvm)) return; - if (kvm_xapic_id(apic) == apic->vcpu->vcpu_id) + /* + * Deliberately truncate the vCPU ID when detecting a modified APIC ID + * to avoid false positives if the vCPU ID, i.e. x2APIC ID, is a 32-bit + * value. + */ + if (kvm_xapic_id(apic) == (u8)apic->vcpu->vcpu_id) return; kvm_set_apicv_inhibit(apic->vcpu->kvm, APICV_INHIBIT_REASON_APIC_ID_MODIFIED);