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Date: Mon, 6 Jun 2022 19:12:48 +0200 From: Ard Biesheuvel <ardb@...nel.org> To: Zhen Lei <thunder.leizhen@...wei.com> Cc: Russell King <linux@...linux.org.uk>, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, Kefeng Wang <wangkefeng.wang@...wei.com>, Linus Walleij <linus.walleij@...aro.org>, Nicolas Pitre <nico@...xnic.net> Subject: Re: [PATCH] ARM: Mark the FDT_FIXED sections as shareable On Mon, 6 Jun 2022 at 17:52, Ard Biesheuvel <ardb@...nel.org> wrote: > > Hello Zhen Lei, > > On Mon, 6 Jun 2022 at 14:49, Zhen Lei <thunder.leizhen@...wei.com> wrote: > > > > commit 7a1be318f579 ("ARM: 9012/1: move device tree mapping out of linear > > region") use FDT_FIXED_BASE to map the whole FDT_FIXED_SIZE memory area > > which contains fdt. But it only reserves the exact physical memory that > > fdt occupied. Unfortunately, this mapping is non-shareable. An illegal or > > speculative read access can bring the RAM content from non-fdt zone into > > cache, PIPT makes it to be hit by subsequently read access through > > shareable mapping(such as linear mapping), and the cache consistency > > between cores is lost due to non-shareable property. > > > > |<---------FDT_FIXED_SIZE------>| > > | | > > ------------------------------- > > | <non-fdt> | <fdt> | <non-fdt> | > > ------------------------------- > > > > 1. CoreA read <non-fdt> through MT_ROM mapping, the old data is loaded > > into the cache. > > 2. CoreB write <non-fdt> to update data through linear mapping. CoreA > > received the notification to invalid the corresponding cachelines, but > > the property non-shareable makes it to be ignored. > > 3. CoreA read <non-fdt> through linear mapping, cache hit, the old data > > is read. > > > > Thanks for the excellent write-up, and for what must have been a lot > of work to narrow down and diagnose! > > > To eliminate this risk, mark the MT_ROM sections as shareable. > > > > The other user of MT_ROM is XIP_KERNEL. XIP allows the kernel to run from > > flash to save RAM space. Not sure if anyone is still using XIP in order to > > save a little memory and not care about performance degradation. Add a new > > memory type MT_ROM_XIP to be compatible with it. > > > > BTW: Another solution is to memblock_reserve() all the sections that fdt > > spans, but this will waste 2-4MiB memory. > > > > I agree that we should not add shareable attributes to the memory type > used by XIP kernels for code regions: NOR flash is not usually > integrated in a way that allows it to participate in the coherency > protocol, so that will likely break things. > > I think, though, that it would be better to leave MT_ROM alone, and > introduce a new type MT_MEMORY_RO instead, which is wired up in the > right way (see below), so that we get NX attributes, and can use it to > create non-section mappings as well. > > Then, as a followup which does not need to go into -stable, we can > reduce the size of the mapping: there is really no need for the > permanent mapping to be section granular - this is only for the early > asm code that is not able to create 2 levels of page tables. > Actually, on second thought, I think reducing the size of the FDT mapping is also needed for correctness, as the non-fdt regions could potentially be covered by a no-map memory reservation, or get mapped non-cacheable for things like non-coherent DMA.
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