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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
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


             reply index

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 ` 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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