Re: [PATCH RESEND v2 2/4] KVM: ensure grow start value is nonzero

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Wed, Nov 06, 2019 at 07:55:00PM +0800, Zhenzhong Duan wrote:
> vcpu->halt_poll_ns could be zeroed in certain cases (e.g. by
> halt_poll_ns = 0). If halt_poll_grow_start is zero,
> vcpu->halt_poll_ns will never be bigger than zero.
> 
> Use param callback to avoid writing zero to halt_poll_grow_start.

This doesn't explain why allowing an admin to disable halt polling by
writing halt_poll_grow_start=0 is a bad thing.  Paolo had the same
question in v1, here[1] and in the guest driver[2].

[1] https://lkml.kernel.org/r/57679389-6e4a-b7ad-559f-3128a608c28a@redhat.com
[2] https://lkml.kernel.org/r/391dd11b-ebbb-28ff-5e57-4a795cd16a1b@redhat.com

> 
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
> ---
>  virt/kvm/kvm_main.c | 22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d6f0696..359516b 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -69,6 +69,26 @@
>  MODULE_AUTHOR("Qumranet");
>  MODULE_LICENSE("GPL");
>  
> +static int grow_start_set(const char *val, const struct kernel_param *kp)
> +{
> +	int ret;
> +	unsigned int n;
> +
> +	if (!val)
> +		return -EINVAL;
> +
> +	ret = kstrtouint(val, 0, &n);
> +	if (ret || !n)
> +		return -EINVAL;
> +
> +	return param_set_uint(val, kp);
> +}
> +
> +static const struct kernel_param_ops grow_start_ops = {
> +	.set = grow_start_set,
> +	.get = param_get_uint,
> +};
> +
>  /* Architectures should define their poll value according to the halt latency */
>  unsigned int halt_poll_ns = KVM_HALT_POLL_NS_DEFAULT;
>  module_param(halt_poll_ns, uint, 0644);
> @@ -81,7 +101,7 @@
>  
>  /* The start value to grow halt_poll_ns from */
>  unsigned int halt_poll_ns_grow_start = 10000; /* 10us */
> -module_param(halt_poll_ns_grow_start, uint, 0644);
> +module_param_cb(halt_poll_ns_grow_start, &grow_start_ops, &halt_poll_ns_grow_start, 0644);
>  EXPORT_SYMBOL_GPL(halt_poll_ns_grow_start);
>  
>  /* Default resets per-vcpu halt_poll_ns . */
> -- 
> 1.8.3.1
>