Re: [PATCH] x86/dom0: Add log for dom0_nodes and dom0_max_vcpus_max conflict

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.02.2022 11:31, Jane Malalane wrote:
> This is not a bug. The xen cmdline can request both a NUMA restriction
> and a vcpu count restriction for Dom0. The node restriction wil always
> be respected which might mean either using dom0_max_vcpus <
> opt_dom0_max_vcpus_max

This is quite normal a case if a range was specified, or did you mean
opt_dom0_max_vcpus_min? But min and max get applied last anyway, so
those always override what was derived from dom0_nr_pxms.

> or using more vCPUs than pCPUs on a node. In
> the case where dom0_max_vcpus gets capped at the maximum number of
> pCPUs for the number of nodes chosen, it can be useful particularly
> for debugging to print a message in the serial log.

The number of vCPU-s Dom0 gets is logged in all cases. And the
reasons why a certain value is uses depends on more than just
the number-of-nodes restriction. I therefor wonder whether the
wording as you've chosen it is potentially misleading, and
properly expressing everything in a single message is going to
be quite a bit too noisy. Furthermore ...

> --- a/xen/arch/x86/dom0_build.c
> +++ b/xen/arch/x86/dom0_build.c
> @@ -240,6 +240,11 @@ unsigned int __init dom0_max_vcpus(void)
>      if ( max_vcpus > limit )
>          max_vcpus = limit;
>  
> +    if ( max_vcpus < opt_dom0_max_vcpus_max && max_vcpus > 
> opt_dom0_max_vcpus_min )
> +        printk(XENLOG_INFO "Dom0 using %d vCPUs conflicts with request to 
> use"
> +               " %d node(s), using up to %d vCPUs\n", opt_dom0_max_vcpus_max,
> +               dom0_nr_pxms, max_vcpus);

... the function can be called more than once, whereas such a
message (if we really want it) would better be issued just once.

To answer your later reply to yourself: I think printk() is fine
here (again assuming we want such a message in the first place);
it's a boot-time-only message after all.

Jan