The real realtime preemption end game

Posted Nov 19, 2023 18:40 UTC (Sun) by mirabilos (subscriber, #84359)
In reply to: The real realtime preemption end game by mirabilos
Parent article: The real realtime preemption end game

Hm, perhaps a bit more elaborating.

Yes, it’s not a persistent storage like the BIOS (or EFI) settings.

No, a boot does not imply memory cleaning (except for memory used during boot, of course). It usually does imply some kind of memory test, and several kinds of memory amount probing by different places in the boot process, but these are often nōn-intrusive enough to keep the memory contents.

A cold boot does have empty memory simply because the memory had no power and the memory controller likewise did not refresh the memory banks.

A warm reboot does not have a period of such, so the memory is *usually* retained.

A hard reboot can fall into either category, depending on how it is executed and wired. The usual power button long-press will be a poweroff followed by a mostly-cold boot; a watchdog reboot, or if the kernel crashed but is still able to reboot-ish (even if just by causing a triple-fault) can be warm reboots (this mostly depends on the memory controller to continue refreshing the memory during that, and of course the firmware not overwriting it).

to post comments

The real realtime preemption end game

Posted Nov 20, 2023 19:57 UTC (Mon) by kreijack (guest, #43513) [Link] (1 responses)

> A warm reboot does not have a period of such, so the memory is *usually* retained.

I think that the key word is "*usually*". On my UEFI system I build a UEFI program that dump the first 4 bytes of the following address:
- 3GB
- 7GB
- 14GB

Then it sets these bytes to a specific value, and then it dump again.

What I saw is:
1) the first time that I run the program, I saw "random values", like 0 and other non 0 values.
2) the 2nd time that I run the program, I saw the same values that I set in the first iteration.

This proof that UEFI doesn't reset the memory between different program invocation.

Then I "warm rebooted" the system, and I saw the "random values" at 1). So it seemed that in my system the memory is cleared between the reboot.

What I'm telling is that at least some bios clears the memory. In may case (a ASUS B550 desktop mainboard) it seems that the BIOS clear the memory.

What I found is that it is possible to force the BIOS to not clear the memory after a reset [1]. But again this is not typically what happens after a crash; after a crash you push the reset physical buttons.

[1] https://stackoverflow.com/questions/36608101/does-a-soft-...

The real realtime preemption end game

Posted Nov 21, 2023 23:31 UTC (Tue) by mirabilos (subscriber, #84359) [Link]

Yes, that’s precisely what I meant with “usually”: a sufficient amount of systems keeps sufficient amounts of memory alive to make this feature worth being in existence, even if counter-examples exist and no spec supports this usage.

The reset button as the only way out of a crash is such a PC thing though. Some machines have watchdogs, and some have something like ddb(4) on BSD or SysRq on Linux that allow for warm reboots even in the face of a crash.