linux-kernel - [PATCH] x86/kvm: Don't use pvqspinlock code if only 1 vCPU

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <1531864767-30648-1-git-send-email-longman@redhat.com>
Date:   Tue, 17 Jul 2018 17:59:27 -0400
From:   Waiman Long <longman@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Joe Mario <jmario@...hat.com>, Waiman Long <longman@...hat.com>
Subject: [PATCH] x86/kvm: Don't use pvqspinlock code if only 1 vCPU

On a VM with only 1 vCPU, the locking fast path will always be
successful. In this case, there is no need to use the the PV qspinlock
code which has higher overhead on the unlock side than the native
qspinlock code.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 arch/x86/kernel/kvm.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 5b2300b..575c9a5 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -748,6 +748,10 @@ void __init kvm_spinlock_init(void)
 	if (kvm_para_has_hint(KVM_HINTS_REALTIME))
 		return;
 
+	/* Don't use the pvqspinlock code if there is only 1 vCPU. */
+	if (num_possible_cpus() == 1)
+		return;
+
 	__pv_init_lock_hash();
 	pv_lock_ops.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
 	pv_lock_ops.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
-- 
1.8.3.1