linux-kernel - [PATCH] KVM: x86: hyper-v: Don't auto-enable stimer during deserialization

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Message-ID: <20231016095217.37574-1-nsaenz@amazon.com>
Date:   Mon, 16 Oct 2023 09:52:17 +0000
From:   Nicolas Saenz Julienne <nsaenz@...zon.com>
To:     <kvm@...r.kernel.org>
CC:     <vkuznets@...hat.com>, <seanjc@...gle.com>, <pbonzini@...hat.com>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
        <dave.hansen@...ux.intel.com>, <x86@...nel.org>, <hpa@...or.com>,
        <graf@...zon.de>, <rkagan@...zon.de>,
        <linux-kernel@...r.kernel.org>,
        Nicolas Saenz Julienne <nsaenz@...zon.com>
Subject: [PATCH] KVM: x86: hyper-v: Don't auto-enable stimer during deserialization

By not honoring the 'stimer->config.enable' state during stimer
deserialization we might introduce spurious timer interrupts. For
example through the following events:
 - 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.
 - We live migrate the VM.
 - 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.

So let's not change the stimer's enable state if the MSR write comes
from user-space.

Fixes: 1f4b34f825e8 ("kvm/x86: Hyper-V SynIC timers")
Signed-off-by: Nicolas Saenz Julienne <nsaenz@...zon.com>
---
 arch/x86/kvm/hyperv.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 7c2dac6824e2..9f1deb6aa131 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -729,7 +729,7 @@ static int stimer_set_count(struct kvm_vcpu_hv_stimer *stimer, u64 count,
 	stimer->count = count;
 	if (stimer->count == 0)
 		stimer->config.enable = 0;
-	else if (stimer->config.auto_enable)
+	else if (stimer->config.auto_enable && !host)
 		stimer->config.enable = 1;
 
 	if (stimer->config.enable)
-- 
2.40.1