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From:   Nicolas Saenz Julienne <nsaenz@amazon.com>
To:     <kvm@vger.kernel.org>
CC:     <vkuznets@redhat.com>, <seanjc@google.com>, <pbonzini@redhat.com>,
        <tglx@linutronix.de>, <mingo@redhat.com>, <bp@alien8.de>,
        <dave.hansen@linux.intel.com>, <x86@kernel.org>, <hpa@zytor.com>,
        <graf@amazon.de>, <rkagan@amazon.de>,
        <linux-kernel@vger.kernel.org>, <anelkz@amazon.de>,
        Nicolas Saenz Julienne <nsaenz@amazon.com>,
        <stable@vger.kernel.org>
Subject: [PATCH v3] KVM: x86: hyper-v: Don't auto-enable stimer on write from user-space
Date:   Tue, 17 Oct 2023 15:51:02 +0000
Message-ID: <20231017155101.40677-1-nsaenz@amazon.com>
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Don't apply the stimer's counter side effects when modifying its
value from user-space, as this may trigger spurious interrupts.

For example:
 - The stimer is configured in auto-enable mode.
 - The stimer's count is set and the timer enabled.
 - The stimer expires, an interrupt is injected.
 - The VM is live migrated.
 - The stimer config and count are deserialized, auto-enable is ON, the
   stimer is re-enabled.
 - The stimer expires right away, and injects an unwarranted interrupt.

Cc: stable@vger.kernel.org
Fixes: 1f4b34f825e8 ("kvm/x86: Hyper-V SynIC timers")
Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>
---

Changes since v2: 
- reword commit message/subject.

Changes since v1:
- Cover all 'stimer->config.enable' updates.

 arch/x86/kvm/hyperv.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 7c2dac6824e2..238afd7335e4 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -727,10 +727,12 @@ static int stimer_set_count(struct kvm_vcpu_hv_stimer *stimer, u64 count,
 
 	stimer_cleanup(stimer);
 	stimer->count = count;
-	if (stimer->count == 0)
-		stimer->config.enable = 0;
-	else if (stimer->config.auto_enable)
-		stimer->config.enable = 1;
+	if (!host) {
+		if (stimer->count == 0)
+			stimer->config.enable = 0;
+		else if (stimer->config.auto_enable)
+			stimer->config.enable = 1;
+	}
 
 	if (stimer->config.enable)
 		stimer_mark_pending(stimer, false);
-- 
2.40.1