Page fault in _mca_init during startup

[Konstantin Belousov]

On Mon, Feb 08, 2021 at 10:03:59AM -0500, Mark Johnston wrote:
> On Mon, Feb 08, 2021 at 12:18:12AM +0200, Konstantin Belousov wrote:
> > On Sun, Feb 07, 2021 at 02:33:11PM -0700, Alan Somers wrote:
> > > Upgrading the BIOS fixed the problem, by clearing the MCG_CMCI_P bit on all
> > > processors.  I don't have strong opinions about whether we should commit
> > > kib's patch too.  Kib, what do you think?
> > 
> > The patch causes some memory over-use.
> > 
> > If this issue is not too widely experienced, I prefer to not commit the patch.
> 
> Couldn't we short-circuit cmci_monitor() if the BSP did not allocate
> anything?
> 
> diff --git a/sys/x86/x86/mca.c b/sys/x86/x86/mca.c
> index 03100e77d45..0619a41b128 100644
> --- a/sys/x86/x86/mca.c
> +++ b/sys/x86/x86/mca.c
> @@ -1070,6 +1070,13 @@ cmci_monitor(int i)
>  
>  	KASSERT(i < mca_banks, ("CPU %d has more MC banks", PCPU_GET(cpuid)));
>  
> +	/*
> +	 * It is possible for some APs to report CMCI support even if the BSP
> +	 * does not, apparently due to a BIOS bug.
> +	 */
> +	if (cmc_state == NULL)
> +		return;
> +
>  	ctl = rdmsr(MSR_MC_CTL2(i));
>  	if (ctl & MC_CTL2_CMCI_EN)
>  		/* Already monitored by another CPU. */
> @@ -1114,6 +1121,10 @@ cmci_resume(int i)
>  
>  	KASSERT(i < mca_banks, ("CPU %d has more MC banks", PCPU_GET(cpuid)));
>  
> +	/* See cmci_monitor(). */
> +	if (cmc_state == NULL)
> +		return;
> +
>  	/* Ignore banks not monitored by this CPU. */
>  	if (!(PCPU_GET(cmci_mask) & 1 << i))
>  		return;
I think something should be printed in this case, at least once.
I believe printf() already works, because spin locks do.