linux-kernel - [PATCH v3 8/8] KVM: Enable virtualization at load/initialization by default

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Date: Fri,  7 Jun 2024 17:06:39 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Chao Gao <chao.gao@...el.com>, Kai Huang <kai.huang@...el.com>
Subject: [PATCH v3 8/8] KVM: Enable virtualization at load/initialization by default

Enable virtualization when KVM loads by default, as doing so avoids the
potential runtime overhead associated with using the cpuhp framework to
enabling virtualization on each CPU.

Prior to commit 10474ae8945c ("KVM: Activate Virtualization On Demand"),
KVM _unconditionally_ enabled virtualization during load, i.e. there's no
fundamental reason KVM needs to dynamically toggle virtualization.  These
days, the only known argument for not enabling virtualization is to allow
KVM to be autoloaded without blocking other out-of-tree hypervisors, and
such use cases can simply change the module param, e.g. via command line.

Note, the aforementioned commit also mentioned that enabling SVM (AMD's
virtualization extensions) can result in "using invalid TLB entries".
It's not clear whether the changelog was referring to a KVM bug, a CPU
bug, or something else entirely.  Regardless, leaving virtualization off
by default is not a robust "fix", as any protection provided is lost the
instant userspace creates the first VM.

Suggested-by: Chao Gao <chao.gao@...el.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 virt/kvm/kvm_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index e20189a89a64..1440c0a7c3c3 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -5495,7 +5495,7 @@ static struct miscdevice kvm_dev = {
 };

 #ifdef CONFIG_KVM_GENERIC_HARDWARE_ENABLING
-static bool enable_virt_at_load;
+static bool enable_virt_at_load = true;
 module_param(enable_virt_at_load, bool, 0444);

 __visible bool kvm_rebooting;
-- 
2.45.2.505.gda0bf45e8d-goog