From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48702) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bn5We-0002qY-QU for qemu-devel@nongnu.org; Thu, 22 Sep 2016 11:03:01 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bn5WY-0002jW-R8 for qemu-devel@nongnu.org; Thu, 22 Sep 2016 11:02:59 -0400 Received: from mx1.redhat.com ([209.132.183.28]:52494) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bn5WY-0002jL-LD for qemu-devel@nongnu.org; Thu, 22 Sep 2016 11:02:54 -0400 Received: from int-mx14.intmail.prod.int.phx2.redhat.com (int-mx14.intmail.prod.int.phx2.redhat.com [10.5.11.27]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 0BFE0AB478 for ; Thu, 22 Sep 2016 15:02:54 +0000 (UTC) From: Paolo Bonzini Date: Thu, 22 Sep 2016 17:02:49 +0200 Message-Id: <1474556571-21200-1-git-send-email-pbonzini@redhat.com> Subject: [Qemu-devel] [PATCH 0/2] kvm: fix incorrect LAPIC reset after reboot in x2apic mode List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: dgilbert@redhat.com Linux 4.8 introduces changes that really really require you to set the APIC base (via either KVM_SET_SREGS or KVM_SET_MSRS) before the KVM_SET_LAPIC ioctl, unless KVM_SET_LAPIC is immediately preceded by KVM_GET_LAPIC. This is because in x2apic mode KVM now moves the APIC id from bits 31-24 of the KVM_SET_LAPIC argument to bits 7-0 of the APIC_ID register (0x20). If QEMU does not nudge the APIC out of x2apic at reset time, the subsequent KVM_SET_LAPIC incorrectly shifts the APIC id right and Linux sees (in bits 31-24) a zero id. While QEMU has always done the right thing, the recent commit 78d6a05 ("x86/lapic: Load LAPIC state at post_load", 2016-09-13) changed that, causing a bug on newer kernels. Fix this by setting the APIC base in kvm_apic_put. Another workaround could be to enable KVM_X2APIC_API_USE_32BIT_IDS, which lets KVM not mangle the APIC ID. Despite the "don't break userspace" mantra, I believe that this can be considered a QEMU bug. I checked kvmtool and it only calls KVM_SET_LAPIC after a KVM_GET_LAPIC, which avoids the issue in the first place. Dr. David Alan Gilbert (1): kvm: apic: set APIC base as part of kvm_apic_put Paolo Bonzini (1): target-i386: introduce kvm_put_one_msr hw/i386/kvm/apic.c | 2 ++ target-i386/kvm.c | 31 ++++++++++++++++++++++--------- target-i386/kvm_i386.h | 2 ++ 3 files changed, 26 insertions(+), 9 deletions(-) -- 2.7.4