[PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

[Andy Lutomirski <luto@kernel.org>]

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