- Special-case NMI handling on the AMD64.
On entry or exit from the kernel the 'alltraps' and 'doreti' the code
used by normal traps disables interrupts to protect the critical sections
where it is setting up %gs.
This protection is insufficient in the presence of NMIs since NMIs
can be taken even when the processor has disabled normal interrupts.
Thus the NMI handler needs to actually read MSR_GBASE on entry to
the kernel to determine whether a swap of %gs using 'swapgs' is
needed. However, reads of MSRs are expensive and integrating this
check into the 'alltraps'/'doreti' path would penalize normal
interrupts.
- Teach DDB about the 'nmi_calltrap' symbol.
Reviewed by: bde, peter (an older version) and ups
Approved by: re (scottl)
fbf27c73ec