Use a dedicated per-CPU stack for machine check exceptions.

Similar to NMIs, machine check exceptions can fire at any time and are
not masked by IF.  This means that machine checks can fire when the
kstack is too deep to hold a trap frame, or at critical sections in
trap handlers when a user %gs is used with a kernel %cs.  Use the same
strategy used for NMIs of using a dedicated per-CPU stack configured
in IST 3.  Store the CPU's pcpu pointer at the stop of the stack so
that the machine check handler can reliably find the proper value for
%gs (also borrowed from NMIs).

This should also fix a similar issue with PTI with a MC# occurring
while the CPU is executing on the trampoline stack.

While here, bypass trap() entirely and just call mca_intr().  This
avoids a bogus call to kdb_reenter() (there's no reason to try to
reenter kdb if a MC# is raised).

Reviewed by:	kib
Tested by:	avg (on AMD without PTI)
Differential Revision:	https://reviews.freebsd.org/D13962

1 files changed, 1 insertions, 1 deletions

diff --git a/sys/amd64/include/intr_machdep.h b/sys/amd64/include/intr_machdep.h
index 47a1a8f19d27..15842696a7a1 100644
--- a/sys/amd64/include/intr_machdep.h
+++ b/sys/amd64/include/intr_machdep.h
@@ -139,7 +139,7 @@ struct trapframe;
 
 /*
  * The following data structure holds per-cpu data, and is placed just
- * above the top of the space used for the NMI stack.
+ * above the top of the space used for the NMI and MC# stacks.
  */
 struct nmi_pcpu {
 	register_t	np_pcpu;