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SubjectRe: [patch, 2.6.10-rc3] safe_hlt() & NMIs

* Linus Torvalds <> wrote:

> The irq window should actually be open every alternate instruction, I
> think. Although it's not actually architected, and I thought that
> there was some errata for some CPU about this..

i have generated an instruction-granularity profile of kernel code
executing the following sequence, driven by the NMI watchdog interrupt:

asm ("cli; cli; sti; cli; sti; cli; sti; cli; sti; cli; sti; ");
asm ("cli; cli; sti; cli; sti; cli; sti; cli; sti; cli; sti; ");
asm ("cli; cli; sti; cli; sti; cli; sti; cli; sti; cli; sti; ");

the first CLI is done twice, to prove that the NMI profiling works and
that the kernel can be interrupted in those places. Then i called this
kernel code in a loop. Here's the result:

c0125ee9: 1529 fa cli
^---------------------------------- # of profiler hits
c0125eea: 507 fb sti
c0125eeb: 0 fa cli
c0125eec: 3719 fb sti
c0125eed: 0 fa cli
c0125eee: 1579 fb sti
c0125eef: 0 fa cli
c0125ef0: 3317 fb sti
c0125ef1: 0 fa cli
c0125ef2: 3030 fb sti
c0125ef3: 0 fa cli
c0125ef4: 2497 fa cli
c0125ef5: 1055 fb sti
c0125ef6: 0 fa cli
c0125ef7: 4674 fb sti
c0125ef8: 0 fa cli
c0125ef9: 3827 fb sti
c0125efa: 0 fa cli
c0125efb: 1622 fb sti
c0125efc: 0 fa cli
c0125efd: 3155 fb sti
c0125efe: 0 fa cli
c0125eff: 1273 fa cli
c0125f00: 512 fb sti
c0125f01: 0 fa cli
c0125f02: 1312 fb sti
c0125f03: 0 fa cli
c0125f04: 1426 fb sti
c0125f05: 0 fa cli
c0125f06: 1507 fb sti
c0125f07: 0 fa cli
c0125f08: 2720 fb sti
c0125f09: 0 fa cli
c0125f0a: 2469 fa cli
c0125f0b: 787 fb sti
c0125f0c: 0 fa cli
c0125f0d: 2085 fb sti
c0125f0e: 0 fa cli
the 'cli' is always a 'black hole' to the NMI, while the second of two
consecutive cli's are not.

i also played a bit with the %ss instructions, and combined them with
the cli/sti instructions and other instructions in various ways, and
with a bit of experimenting found the following, somewhat surprising

c0125f33: 1016 66 8c d0 mov %ss,%ax
c0125f36: 6626 8e d0 mov %eax,%ss
c0125f38: 34715 8e d0 mov %eax,%ss
c0125f3a: 14682 8e d0 mov %eax,%ss
c0125f3c: 4521 8e d0 mov %eax,%ss
c0125f3e: 7564 8e d0 mov %eax,%ss
c0125f40: 3861 66 8e d0 mov %ax,%ss
c0125f43: 0 66 8c d1 mov %ss,%cx
c0125f46: 1061 66 8c da mov %ds,%dx
c0125f49: 7660 8e d1 mov %ecx,%ss
c0125f4b: 11322 17 pop %ss
c0125f4c: 0 fb sti
c0125f4d: 8935 8e d1 mov %ecx,%ss
c0125f4f: 0 fa cli
c0125f50: 2198 66 8c d1 mov %ss,%cx
c0125f53: 735 66 8c da mov %ds,%dx
c0125f56: 0 8e da mov %edx,%ds
c0125f58: 6400 8e d0 mov %eax,%ss
c0125f5a: 3062 8e d0 mov %eax,%ss
c0125f5c: 3552 8e d0 mov %eax,%ss
c0125f5e: 4818 8e d0 mov %eax,%ss
c0125f60: 0 fb sti
c0125f61: 0 66 8c da mov %ds,%dx
c0125f64: 17788 8e d0 mov %eax,%ss
c0125f66: 64694 8e d0 mov %eax,%ss
c0125f68: 12837 8e d0 mov %eax,%ss
c0125f6a: 9859 8e d0 mov %eax,%ss
c0125f6c: 0 fb sti
c0125f6d: 74506 8e d0 mov %eax,%ss
c0125f6f: 0 fb sti
c0125f70: 8589 fa cli
c0125f71: 10248 8e d0 mov %eax,%ss
c0125f73: 3825 8e d0 mov %eax,%ss
c0125f75: 4903 8e d0 mov %eax,%ss
c0125f77: 71134 8e d0 mov %eax,%ss
c0125f79: 0 fb sti
c0125f7a: 0 fa cli
c0125f7b: 7461 8e d0 mov %eax,%ss
c0125f7d: 0 66 8c d0 mov %ss,%ax
c0125f80: 39387 8e d0 mov %eax,%ss
c0125f82: 0 fa cli
c0125f83: 41484 8e d0 mov %eax,%ss
c0125f85: 0 fa cli
c0125f86: 4490 8e d0 mov %eax,%ss
c0125f88: 0 fa cli
c0125f89: 6024 8e d0 mov %eax,%ss
c0125f8b: 15454 8e d0 mov %eax,%ss
c0125f8d: 0 fb sti
c0125f8e: 0 fb sti
c0125f8f: 115104 fb sti
c0125f90: 39061 fb sti

it shows a number of interesting effects:

- "mov %eax,%ss" followed by the _same_ instruction cancels the
black-hole. This i suspect is done to prevent the lockup in vm86

- an %ss black-hole instruction followed by 'sti' cancels sti's
black-hole. This is unlikely to occur in real kernel code, but we
might want to add a 'nop' in front of safe_halt()'s sti, to make sure
the black-hole takes effect.

- in one case a two-instruction blackhole was created - but this might
be some prefetch effect.

i played around with the instructions a bit to manufacture combinations
that enlengthen the black-hole but failed :) This was on an Athlon64.

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