linux-kernel - [PATCH 3/3] KVM: x86: WARN only once if KVM leaves a dangling userspace I/O request

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Message-Id: <20220711232750.1092012-4-seanjc@google.com>
Date:   Mon, 11 Jul 2022 23:27:50 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+760a73552f47a8cd0fd9@...kaller.appspotmail.com,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Hou Wenlong <houwenlong.hwl@...group.com>
Subject: [PATCH 3/3] KVM: x86: WARN only once if KVM leaves a dangling
 userspace I/O request

Change a WARN_ON() to separate WARN_ON_ONCE() if KVM has an outstanding
PIO or MMIO request without an associated callback, i.e. if KVM queued a
userspace I/O exit but didn't actually exit to userspace before moving
on to something else.  Warning on every KVM_RUN risks spamming the kernel
if KVM gets into a bad state.  Opportunistically split the WARNs so that
it's easier to triage failures when a WARN fires.

Deliberately do not use KVM_BUG_ON(), i.e. don't kill the VM.  While the
WARN is all but guaranteed to fire if and only if there's a KVM bug, a
dangling I/O request does not present a danger to KVM (that flag is truly
truly consumed only in a single emulator path), and any such bug is
unlikely to be fatal to the VM (KVM essentially failed to do something it
shouldn't have tried to do in the first place).  In other words, note the
bug, but let the VM keep running.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/x86.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 567d13405445..50dc55996416 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10847,8 +10847,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
 		r = cui(vcpu);
 		if (r <= 0)
 			goto out;
-	} else
-		WARN_ON(vcpu->arch.pio.count || vcpu->mmio_needed);
+	} else {
+		WARN_ON_ONCE(vcpu->arch.pio.count);
+		WARN_ON_ONCE(vcpu->mmio_needed);
+	}

 	if (kvm_run->immediate_exit) {
 		r = -EINTR;
-- 
2.37.0.144.g8ac04bfd2-goog