linux-kernel - [PATCH] kvm: cap halt polling at exactly halt_poll

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Message-Id: <1457482784-82531-1-git-send-email-dmatlack@google.com>
Date:	Tue,  8 Mar 2016 16:19:44 -0800
From:	David Matlack <dmatlack@...gle.com>
To:	kvm@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, pbonzini@...hat.com,
	wanpeng.li@...mail.com, David Matlack <dmatlack@...gle.com>
Subject: [PATCH] kvm: cap halt polling at exactly halt_poll_ns

When growing halt-polling, there is no check that the poll time exceeds
the limit. It's possible for vcpu->halt_poll_ns grow once past
halt_poll_ns, and stay there until a halt which takes longer than
vcpu->halt_poll_ns. For example, booting a Linux guest with
halt_poll_ns=11000:

 ... kvm:kvm_halt_poll_ns: vcpu 0: halt_poll_ns 0 (shrink 10000)
 ... kvm:kvm_halt_poll_ns: vcpu 0: halt_poll_ns 10000 (grow 0)
 ... kvm:kvm_halt_poll_ns: vcpu 0: halt_poll_ns 20000 (grow 10000)

Signed-off-by: David Matlack <dmatlack@...gle.com>
---
 virt/kvm/kvm_main.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index a11cfd2..9102ae1 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1952,6 +1952,9 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 	else
 		val *= halt_poll_ns_grow;
 
+	if (val > halt_poll_ns)
+		val = halt_poll_ns;
+
 	vcpu->halt_poll_ns = val;
 	trace_kvm_halt_poll_ns_grow(vcpu->vcpu_id, val, old);
 }
-- 
2.7.0.rc3.207.g0ac5344