Re: [PATCH] KVM: x86: Make the debugfs per VM optional

[Sean Christopherson <seanjc@google.com>]

+Paolo and others

Please use scripts/get_maintainer.pl, otherwise your patches are likely to be
missed by key folks (Paolo, in this case).

On Wed, Oct 23, 2024, Bernhard Kauer wrote:
> Creating a debugfs directory for each virtual machine is a suprisingly
> costly operation as one has to synchronize multiple cores. However, short
> living VMs seldom benefit from it.
> 
> Since there are valid use-cases we make this feature optional via a
> module parameter. Disabling it saves 150us in the hello microbenchmark.
> 
> Signed-off-by: Bernhard Kauer <bk@alpico.io>
> ---
>  virt/kvm/kvm_main.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index a48861363649..760e39cf86a8 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -94,6 +94,9 @@ unsigned int halt_poll_ns_shrink = 2;
>  module_param(halt_poll_ns_shrink, uint, 0644);
>  EXPORT_SYMBOL_GPL(halt_poll_ns_shrink);
>  
> +bool debugfs_per_vm = true;
> +module_param(debugfs_per_vm, bool, 0644);

I'm not opposed to letting userspace say "no debugfs for me", but I don't know
that a module param is the right way to go.  It's obviously quite easy to
implement and maintain (in code), but I'm mildly concerned that it'll have limited
usefulness and/or lead to bad user experiences, e.g. because people turn off debugfs
for startup latency without entirely realizing what they're sacrificing.

One potentially terrible idea would be to setup debugfs asynchronously, so that
the VM is runnable asap, but userspace still gets full debugfs information.  The
two big wrinkles would be the vCPU debugfs creation and kvm_uevent_notify_change()
(or at least the STATS_PATH event) would both need to be asynchronous as well.