Re: [PATCH v2] powerpc: warn on emulation of dcbz instruction in kernel mode
From: Christoph Hellwig <hch@lst.de>
Date: 2024-08-23 13:55:03
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On Fri, Aug 23, 2024 at 08:06:00AM -0500, Segher Boessenkool wrote:
What does "uncached memory" even mean here? Literally it would be
I=1 memory (uncachEABLE memory), but more likely you want M=0 memory
here ("non-memory memory", "not well-behaved memory", MMIO often).Regular kernel memory vmapped with pgprot_noncached().
If memset() is expected to be used with M=0, you cannot do any serious optimisations to it at all. If memset() is expected to be used with I=1 it should use a separate code path for it, probably the caller should make the distinction.
DMA coherent memory which uses uncached memory for platforms that do not provide hardware dma coherence can end up just about anywhere in the kernel. We could use special routines for a few places in the DMA subsystem, but there might be plenty of others.