Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756535Ab2K2BA5 (ORCPT ); Wed, 28 Nov 2012 20:00:57 -0500 Received: from e5.ny.us.ibm.com ([32.97.182.145]:42617 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756238Ab2K2BA4 (ORCPT ); Wed, 28 Nov 2012 20:00:56 -0500 Subject: [RFC][PATCH 2/2] fix kvm's use of __pa() on percpu areas To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, Gleb Natapov , Avi Kivity , Dave Hansen From: Dave Hansen Date: Thu, 29 Nov 2012 00:53:56 +0000 References: <20121129005355.966BD487@kernel.stglabs.ibm.com> In-Reply-To: <20121129005355.966BD487@kernel.stglabs.ibm.com> Message-Id: <20121129005356.186166B0@kernel.stglabs.ibm.com> X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12112900-5930-0000-0000-00000EC2527A Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3100 Lines: 80 In short, it is illegal to call __pa() on an address holding a percpu variable. The times when this actually matters are pretty obscure (certain 32-bit NUMA systems), but it _does_ happen. It is important to keep KVM guests working on these systems because the real hardware is getting harder and harder to find. This bug manifested first by me seeing a plain hang at boot after this message: CPU 0 irqstacks, hard=f3018000 soft=f301a000 or, sometimes, it would actually make it out to the console: [ 0.000000] BUG: unable to handle kernel paging request at ffffffff I eventually traced it down to the KVM async pagefault code. This can be worked around by disabling that code either at compile-time, or on the kernel command-line. The kvm async pagefault code was injecting page faults in to the guest which the guest misinterpreted because its "reason" was not being properly sent from the host. The guest passes a physical address of an per-cpu async page fault structure via an MSR to the host. Since __pa() is broken on percpu data, the physical address it sent was bascially bogus and the host went scribbling on random data. The guest never saw the real reason for the page fault (it was injected by the host), assumed that the kernel had taken a _real_ page fault, and panic()'d. Signed-off-by: Dave Hansen --- linux-2.6.git-dave/arch/x86/kernel/kvm.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff -puN arch/x86/kernel/kvm.c~fix-kvm-__pa-use-on-percpu-areas arch/x86/kernel/kvm.c --- linux-2.6.git/arch/x86/kernel/kvm.c~fix-kvm-__pa-use-on-percpu-areas 2012-11-29 00:39:59.130213376 +0000 +++ linux-2.6.git-dave/arch/x86/kernel/kvm.c 2012-11-29 00:51:55.428091802 +0000 @@ -284,9 +284,9 @@ static void kvm_register_steal_time(void memset(st, 0, sizeof(*st)); - wrmsrl(MSR_KVM_STEAL_TIME, (__pa(st) | KVM_MSR_ENABLED)); + wrmsrl(MSR_KVM_STEAL_TIME, (slow_virt_to_phys(st) | KVM_MSR_ENABLED)); printk(KERN_INFO "kvm-stealtime: cpu %d, msr %lx\n", - cpu, __pa(st)); + cpu, slow_virt_to_phys(st)); } static DEFINE_PER_CPU(unsigned long, kvm_apic_eoi) = KVM_PV_EOI_DISABLED; @@ -311,7 +311,7 @@ void __cpuinit kvm_guest_cpu_init(void) return; if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF) && kvmapf) { - u64 pa = __pa(&__get_cpu_var(apf_reason)); + u64 pa = slow_virt_to_phys(&__get_cpu_var(apf_reason)); #ifdef CONFIG_PREEMPT pa |= KVM_ASYNC_PF_SEND_ALWAYS; @@ -327,7 +327,8 @@ void __cpuinit kvm_guest_cpu_init(void) /* Size alignment is implied but just to make it explicit. */ BUILD_BUG_ON(__alignof__(kvm_apic_eoi) < 4); __get_cpu_var(kvm_apic_eoi) = 0; - pa = __pa(&__get_cpu_var(kvm_apic_eoi)) | KVM_MSR_ENABLED; + pa = slow_virt_to_phys(&__get_cpu_var(kvm_apic_eoi)) + | KVM_MSR_ENABLED; wrmsrl(MSR_KVM_PV_EOI_EN, pa); } _ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/