Further investigation shows that pause may be working, but very very
slowly.
The "use-case" in NetBSD is for "hatching" application CPUs. The target
CPU runs a loop that does
while (flag_1 not set)
for (i = 0; i < 10000; i++)
x86_pause(); /* which is assembly code: "pause;
ret;" */
...
set flag_2;
return;
The boot CPU executes the following code for each application CPU:
set flag_1;
for (i = 0; i < 100000 && flag_2 not set; i++)
i8254_delay(10); /* this should be 10usec per iteration,
1.0 sec total */
if (flag_2 not set)
panic(cpu did not hatch);
....
So, the 10k executions of x86_pause are taking in excess of 1 sec to
complete. Indeed, reducing that constant value from 10k to only 100
results in the same failure. So it would seem that the pause
instruction is taking an extremely long time to complete. (Further
reducing the interation count to only 50 results in success, although it
"feels" very sluggish.)
--
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https://bugs.launchpad.net/bugs/1693649
Title:
x86 pause misbehaves with -cpu haswell
Status in QEMU:
New
Bug description:
Using qemu-2.9.0
When booting NetBSD using '-cpu haswell -smp 4', the system fails to
initialize the additional CPUs. It appears as though the "application
processor" enters routine x86_pause() but never returns.
x86_pause() is simply two assembler instructions: 'pause; ret;'
Replacing the routine with 'nop; nop; ret;' allows the system to
proceed, of course without the benefit of the pause instruction on
spin-loops!
Additionally, booting with '-cpu phenom -smp 4' also works, although
the system does seem confused about the type of CPU being used.
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