How can it run when there is no MMU? Isn't this like rewriting a large part of the kernel?

This is for the recently released ESP32-S31 which does have a MMU, unlike the ESP32-S3.

The author has more details in this reddit post: https://eddrit.com/r/esp32/comments/1vait52/mmu_linux_on_the... And the docs section of the repo: https://github.com/GrieferPig/esp32-s31-linux/tree/main/docs...

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Rochus 5 hours ago | parent | next [-]

Thanks for the links. This doesn't seem to be a true RISC-V MMU (according to the Sv32 specification) integrated into the CPU core itself, but just a peripheral designed for memory mapped SPI flash and PSRAM. So as far as I understand there is no true process isolation with page faults and dynamic paging.

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Rohansi 3 hours ago | parent [-]

Sv32 is what every 32-bit RISC-V CPU with an MMU uses. It is a full MMU. You can run Linux on it.

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Rochus 3 hours ago | parent [-]

Sure, but what the S31 calls "MMU" is not an Sv32 MMU; therefore my comment.

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Rohansi 2 hours ago | parent [-]

The documentation states:

> Compliant with RISC-V Sv32 virtual memory scheme

https://documentation.espressif.com/esp32-s31_datasheet_en.p...

	▲	Rochus 35 minutes ago \| parent \| next [-]
		Ok, I see. The S3’s "MMU" is just an external-memory mapper, not a virtual-memory MMU. The S31 apparently has both, that mapper plus an architectural CPU-side Sv32 MMU; that offers indeed a lot of interesting possibilities.
	▲	2 hours ago \| parent \| prev [-]
		[deleted]

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chrsw 5 hours ago | parent | prev [-]

The Microchip PIC32MZ MCU has an MMU as well. But not with wireless options in a 8x8 QFN80 package like this ESP32-S31. That's pretty small. No flash though.

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yjftsjthsd-h 6 hours ago | parent | prev [-]

There is actually precedent for nommu Linux, though it obviously has tradeoffs.

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stevefan1999 3 hours ago | parent [-]

Yeah, nommu Linux basically cannot run normal ELF since there is no virtual memory which is needed for relative addressing and relocations. You are mostly left with classical formats like AT&T a.out only

	▲	derefr 2 hours ago \| parent [-]
		You wouldn’t be able to guarantee execution of arbitrary ELF, but couldn’t you intentionally build non-PIC-compiled ELF executables where the section base addresses as defined in the header must match the MMU region “slots” the host provides?