[ipxe-devel] [PATCH 1/1] efi: provide possibility to disable cpu_nap

[Michael Brown]

On 13/07/17 06:50, Heinrich Schuchardt wrote:
>> You could add code in efiarm_nap.c and/or efix86_nap.c to detect whether
>> or not it is safe to sleep the CPU.  The attached (completely untested)
>> patch demonstrates this idea: modify the cpu_nap() implementation in
>> efix86_nap.c to execute "hlt" only if the platform firmware has enabled
>> interrupts.
> 
> By analyzing CPU registers it is not possible to find out if the
> external hardware is set up to trigger interrupts.

It allows the platform firmware (such as U-Boot) to deliberately signal 
to other programs (such as UEFI iPXE) that interrupts are unexpectedly 
disabled.  You would have to ensure that U-Boot actually does this.

Disabling interrupts via "cli" will not prevent CPU exceptions such as 
page faults on x86.  The Intel SDM states:

   "Clearing the IF flag causes the processor to ignore maskable
    external interrupts.  The IF flag and the CLI and STI instruction
    have no affect on the generation of exceptions and NMI interrupts."

I assume that the equivalent is true on ARM, but you should check the 
documentation to be sure.

> What is wrong about deciding at compile time if we enable cpu_nap()?

The "#ifdef MY_PET_FEATURE" anti-pattern has been banned to the maximum 
extent possible from new iPXE code since around 2005 (before the name 
change from Etherboot to gPXE, let alone iPXE).  See

   http://dox.ipxe.org/ifdef_harmful.html

for a rationale.

Michael