From: Andy Lutomirski <luto@kernel.org>
To: LKML <linux-kernel@vger.kernel.org>,
x86@kernel.org, kvm list <kvm@vger.kernel.org>,
Paolo Bonzini <pbonzini@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>, stable@vger.kernel.org
Subject: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS
Date: Fri, 6 Mar 2020 18:26:42 -0800 [thread overview]
Message-ID: <ed71d0967113a35f670a9625a058b8e6e0b2f104.1583547991.git.luto@kernel.org> (raw)
The ABI is broken and we cannot support it properly. Turn it off.
If this causes a meaningful performance regression for someone, KVM
can introduce an improved ABI that is supportable.
Cc: stable@vger.kernel.org
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/kernel/kvm.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 93ab0cbd304e..e6f2aefa298b 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -318,11 +318,26 @@ static void kvm_guest_cpu_init(void)
pa = slow_virt_to_phys(this_cpu_ptr(&apf_reason));
-#ifdef CONFIG_PREEMPTION
- pa |= KVM_ASYNC_PF_SEND_ALWAYS;
-#endif
pa |= KVM_ASYNC_PF_ENABLED;
+ /*
+ * We do not set KVM_ASYNC_PF_SEND_ALWAYS. With the current
+ * KVM paravirt ABI, the following scenario is possible:
+ *
+ * #PF: async page fault (KVM_PV_REASON_PAGE_NOT_PRESENT)
+ * NMI before CR2 or KVM_PF_REASON_PAGE_NOT_PRESENT
+ * NMI accesses user memory, e.g. due to perf
+ * #PF: normal page fault
+ * #PF reads CR2 and apf_reason -- apf_reason should be 0
+ *
+ * outer #PF reads CR2 and apf_reason -- apf_reason should be
+ * KVM_PV_REASON_PAGE_NOT_PRESENT
+ *
+ * There is no possible way that both reads of CR2 and
+ * apf_reason get the correct values. Fixing this would
+ * require paravirt ABI changes.
+ */
+
if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_VMEXIT))
pa |= KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;
--
2.24.1
next reply other threads:[~2020-03-07 2:26 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-07 2:26 Andy Lutomirski [this message]
2020-03-07 15:03 ` [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS Andy Lutomirski
2020-03-07 15:47 ` Thomas Gleixner
2020-03-07 15:59 ` Andy Lutomirski
2020-03-07 19:01 ` Thomas Gleixner
2020-03-07 19:34 ` Andy Lutomirski
2020-03-08 7:23 ` Thomas Gleixner
2020-03-09 6:57 ` Thomas Gleixner
2020-03-09 8:40 ` Paolo Bonzini
2020-03-09 9:09 ` Thomas Gleixner
2020-03-09 18:14 ` Andy Lutomirski
2020-03-09 19:05 ` Thomas Gleixner
2020-03-09 20:22 ` Peter Zijlstra
2020-04-06 19:09 ` Vivek Goyal
2020-04-06 20:25 ` Peter Zijlstra
2020-04-06 20:32 ` Andy Lutomirski
2020-04-06 20:42 ` Andy Lutomirski
2020-04-07 17:21 ` Vivek Goyal
2020-04-07 17:38 ` Andy Lutomirski
2020-04-07 20:20 ` Thomas Gleixner
2020-04-07 21:41 ` Andy Lutomirski
2020-04-07 22:07 ` Paolo Bonzini
2020-04-07 22:29 ` Andy Lutomirski
2020-04-08 0:30 ` Paolo Bonzini
2020-05-21 15:55 ` Vivek Goyal
2020-04-07 22:48 ` Thomas Gleixner
2020-04-08 4:48 ` Andy Lutomirski
2020-04-08 9:32 ` Borislav Petkov
2020-04-08 10:12 ` Thomas Gleixner
2020-04-08 18:23 ` Vivek Goyal
2020-04-07 22:49 ` Vivek Goyal
2020-04-08 10:01 ` Borislav Petkov
2020-04-07 22:04 ` Paolo Bonzini
2020-04-07 23:21 ` Thomas Gleixner
2020-04-08 8:23 ` Paolo Bonzini
2020-04-08 13:01 ` Thomas Gleixner
2020-04-08 15:38 ` Peter Zijlstra
2020-04-08 16:41 ` Thomas Gleixner
2020-04-09 9:03 ` Paolo Bonzini
2020-04-08 15:34 ` Sean Christopherson
2020-04-08 16:50 ` Paolo Bonzini
2020-04-08 18:01 ` Thomas Gleixner
2020-04-08 20:34 ` Vivek Goyal
2020-04-08 23:06 ` Thomas Gleixner
2020-04-08 23:14 ` Thomas Gleixner
2020-04-09 4:50 ` Andy Lutomirski
2020-04-09 9:43 ` Paolo Bonzini
2020-04-09 11:36 ` Andrew Cooper
2020-04-09 12:47 ` Paolo Bonzini
2020-04-09 14:13 ` Andrew Cooper
2020-04-09 14:32 ` Paolo Bonzini
2020-04-09 15:03 ` Andy Lutomirski
2020-04-09 15:17 ` Paolo Bonzini
2020-04-09 17:32 ` Andy Lutomirski
2020-04-06 21:32 ` Thomas Gleixner
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