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Wed, 14 Aug 2024 11:20:56 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGz3KdZ+hT/xudOLocHBKfqm0syJCiNBfzo5Wx1XSl+fmJNsfepKFF8yyxqa5f0vgg3nSVZgA== X-Received: by 2002:a05:600c:4e4d:b0:426:59fe:ac27 with SMTP id 5b1f17b1804b1-429dd25f730mr31814835e9.26.1723659655883; Wed, 14 Aug 2024 11:20:55 -0700 (PDT) Received: from [192.168.10.47] ([151.95.101.29]) by smtp.googlemail.com with ESMTPSA id 5b1f17b1804b1-429d877e066sm62720955e9.1.2024.08.14.11.20.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 14 Aug 2024 11:20:54 -0700 (PDT) Message-ID: <5de9e883-e93b-4f50-b926-ac25613fe0c4@redhat.com> Date: Wed, 14 Aug 2024 20:20:53 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 8/8] KVM: Enable virtualization at load/initialization by default To: Sean Christopherson Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Chao Gao , Kai Huang References: <20240608000639.3295768-1-seanjc@google.com> <20240608000639.3295768-9-seanjc@google.com> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 6/8/24 02:06, Sean Christopherson wrote: > 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. Hah, should have read ahead to this point of the series. Just squash this in the earlier patch and call it a day; and place all the worrisome remarks about possible latency along with documentation for the parameter. Oops, there's no documentation, :) please add it to Documentation/admin-guide/kernel-parameters.txt in v2. Thanks, Paolo > 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 > Signed-off-by: Sean Christopherson > --- > 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;