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

["Huang, Kai" <kai.huang@intel.com>]

On 24/05/2024 2:39 pm, Chao Gao wrote:
> On Fri, May 24, 2024 at 11:11:37AM +1200, Huang, Kai wrote:
>>
>>
>> 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.
> 
> Enabling virtualization at runtime was added to support an off-tree hypervisor
> (see the commit below).  

Oh, ok.  I was thinking something else.

>>
>> 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.
> 
> IIUC, we plan to set up TDX module at KVM load time; we need to enable virt
> at load time at least for TDX. Definitely, on_each_cpu() can solve the perf
> concern. But a solution which can also satisfy TDX's need is better to me.
> 

Doing on_each_cpu() explicitly can also meet TDX's purpose.  We just 
explicitly enable virtualization during module loading if we are going 
to enable TDX.  For all other cases, the behaivour remains the same, 
unless they want to change when to enable virtualization, e.g., when 
loading module by default.

For always, or by default enabling virtualization during module loading, 
we somehow discussed before:

https://lore.kernel.org/kvm/ZiKoqMk-wZKdiar9@google.com/

My true comment is introducing a module parameter, which is a userspace 
ABI, to just fix some performance downgrade seems overkill when it can 
be mitigated by the kernel.