linux-kernel - Re: [PATCH v2 3/6] KVM: Add a module param to allow enabling virtualization when KVM is loaded

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Date: Fri, 24 May 2024 11:11:37 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: Chao Gao <chao.gao@...el.com>
CC: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
	<pbonzini@...hat.com>, <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/6] KVM: Add a module param to allow enabling
 virtualization when KVM is loaded



On 23/05/2024 4:23 pm, Chao Gao wrote:
> On Thu, May 23, 2024 at 10:27:53AM +1200, Huang, Kai wrote:
>>
>>
>> On 22/05/2024 2:28 pm, Sean Christopherson wrote:
>>> Add an off-by-default module param, enable_virt_at_load, to let userspace
>>> force virtualization to be enabled in hardware when KVM is initialized,
>>> i.e. just before /dev/kvm is exposed to userspace.  Enabling virtualization
>>> during KVM initialization allows userspace to avoid the additional latency
>>> when creating/destroying the first/last VM.  Now that KVM uses the cpuhp
>>> framework to do per-CPU enabling, the latency could be non-trivial as the
>>> cpuhup bringup/teardown is serialized across CPUs, e.g. the latency could
>>> be problematic for use case that need to spin up VMs quickly.
>>
>> How about we defer this until there's a real complain that this isn't
>> acceptable?  To me it doesn't sound "latency of creating the first VM"
>> matters a lot in the real CSP deployments.
> 
> I suspect kselftest and kvm-unit-tests will be impacted a lot because
> hundreds of tests are run serially. And it looks clumsy to reload KVM
> module to set enable_virt_at_load to make tests run faster. I think the
> test slowdown is a more realistic problem than running an off-tree
> hypervisor, so I vote to make enabling virtualization at load time the
> default behavior and if we really want to support an off-tree hypervisor,
> we can add a new module param to opt in enabling virtualization at runtime.

I am not following why off-tree hypervisor is ever related to this.

Could you elaborate?

The problem of enabling virt during module loading by default is it 
impacts all ARCHs.  Given this performance downgrade (if we care) can be 
resolved by explicitly doing on_each_cpu() below, I am not sure why we 
want to choose this radical approach.


>> Or we just still do:
>>
>> 	cpus_read_lock();
>> 	on_each_cpu(hardware_enable_nolock, ...);
>> 	cpuhp_setup_state_nocalls_cpuslocked(...);
>> 	cpus_read_unlock();
>>
>> I think the main benefit of series is to put all virtualization enabling
>> related things into one single function.  Whether using cpuhp_setup_state()
>> or using on_each_cpu() shouldn't be the main point.
>>