Re: [RFC PATCH 1/3] xl: replace vcpu-set --ignore-host with --ignore-warn

[George Dunlap <george.dunlap@eu.citrix.com>]

On 07/19/2013 04:48 PM, Konrad Rzeszutek Wilk wrote:
> When Xen 4.3 was released we had a discussion whether we should
> allow the vcpu-set command to allow the user to set more than
> physical CPUs for a guest. The author brought up:
>   - Xend used to do it,
>   - If a user wants to do it, let them do it,
>   - The original author of the change did not realize the
>     side-effect his patch caused this and had no intention of changing it.
>   - The user can already boot a massively overcommitted guest by
>     having a large 'vcpus=' value in the guest config and we allow
>     that.
>
> Since we were close to the release we added --ignore-host parameter
> as a mechanism for a user to still set more vCPUs that the physical
> machine as a stop-gate.
>
> This patch removes said option and adds the --ignore-warn option.
> By default the user is allowed to set as many vCPUs as they would like.
> We will print out a warning if the value is higher than the physical
> CPU count. The --ignore-warn will silence said warning.

I think this is a good change in general, but I don't think the name is 
quite right.  You're not ignoring the warnings, you're turning them off. 
  Maybe make the function argument "warn", and the option "--no-warn"?

> +    host_cpu = libxl_get_max_cpus(ctx);
> +    if (max_vcpus > host_cpu && !ignore_warn) {
> +        fprintf(stderr, "WARNING: You are overcommmitting! You have %d" \
> +                " physical CPUs and want %d vCPUs! Continuing, use" \
> +                " --ignore-warn to silence this.\n", host_cpu, max_vcpus);
>       }

This is relatively minor, but you might as well keep the same basic 
structure here, and only call libxl_get_max_cpus() if warn is set.

  -George