Page fault in _mca_init during startup

[Mark Johnston]

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;