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Date: Thu, 2 Apr 2020 13:17:45 +0100
From: Mark Rutland <>
To: Catalin Marinas <>
Cc: Ard Biesheuvel <>, Will Deacon <>,
	Linux ARM <>,
Subject: Re: [RFC PATCH] arm64: remove CONFIG_DEBUG_ALIGN_RODATA feature

On Thu, Apr 02, 2020 at 12:30:33PM +0100, Catalin Marinas wrote:
> On Mon, Mar 30, 2020 at 04:32:31PM +0200, Ard Biesheuvel wrote:
> > Could we make this a runtime thing? E.g., remap the entire linear
> > region down to pages under stop_machine() the first time we probe a
> > device that uses non-coherent DMA?
> That could be pretty expensive at run-time. With the ARMv8.4-TTRem
> feature, I wonder whether we could do this lazily when allocating
> non-coherent DMA buffers.

It's worth noting that ARMv8.4-TTRem is optional and the "level 1" and
"level 2" behaviours still allows non-fatal faults to be taken while nT
is set (or until you perform the TLB invalidation). We can only safely
use it where we could use a full BBM sequence today.

Effectively TTRem is an optimized, where the CPU *might* be able to use
entries during the break stage, but is not guaranteed to do so.


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