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Date:   Fri,  6 Mar 2020 18:16:27 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>, stable@...r.kernel.org
Subject: [PATCH] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

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@...r.kernel.org
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/kernel/kvm.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 93ab0cbd304e..71f9f39f93da 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -318,11 +318,16 @@ 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, if an async page fault occurs on an early
+		 * memory access in the normal (sync) #PF path or in an NMI
+		 * that happens early in the #PF code, the combination of CR2
+		 * and the APF reason field will be corrupted.
+		 */
+
 		if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_VMEXIT))
 			pa |= KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;
 
-- 
2.24.1

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