From ce64d61801d9fef31350e65749e6cdbf53df6409 Mon Sep 17 00:00:00 2001
From: Sergey Senozhatsky <senozhatsky@chromium.org>
Date: Thu, 2 Sep 2021 12:11:00 +0900
Subject: [PATCH] KVM: do not shrink halt_poll_ns below grow_start
Git-commit: ae232ea460888dc5a8b37e840c553b02521fbf18
Patch-mainline: v5.15-rc4
References: stable-5.14.11

[ Upstream commit ae232ea460888dc5a8b37e840c553b02521fbf18 ]

grow_halt_poll_ns() ignores values between 0 and
halt_poll_ns_grow_start (10000 by default). However,
when we shrink halt_poll_ns we may fall way below
halt_poll_ns_grow_start and endup with halt_poll_ns
values that don't make a lot of sense: like 1 or 9,
or 19.

VCPU1 trace (halt_poll_ns_shrink equals 2):

VCPU1 grow 10000
VCPU1 shrink 5000
VCPU1 shrink 2500
VCPU1 shrink 1250
VCPU1 shrink 625
VCPU1 shrink 312
VCPU1 shrink 156
VCPU1 shrink 78
VCPU1 shrink 39
VCPU1 shrink 19
VCPU1 shrink 9
VCPU1 shrink 4

Mirror what grow_halt_poll_ns() does and set halt_poll_ns
to 0 as soon as new shrink-ed halt_poll_ns value falls
below halt_poll_ns_grow_start.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: <20210902031100.252080-1-senozhatsky@chromium.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 virt/kvm/kvm_main.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index b50dbe269f4b..1a11dcb670a3 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3053,15 +3053,19 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 
 static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
 {
-	unsigned int old, val, shrink;
+	unsigned int old, val, shrink, grow_start;
 
 	old = val = vcpu->halt_poll_ns;
 	shrink = READ_ONCE(halt_poll_ns_shrink);
+	grow_start = READ_ONCE(halt_poll_ns_grow_start);
 	if (shrink == 0)
 		val = 0;
 	else
 		val /= shrink;
 
+	if (val < grow_start)
+		val = 0;
+
 	vcpu->halt_poll_ns = val;
 	trace_kvm_halt_poll_ns_shrink(vcpu->vcpu_id, val, old);
 }
-- 
2.26.2