Messages in this thread | | | Subject | Re: [PATCH] ARM: Mark the FDT_FIXED sections as shareable | From | "Leizhen (ThunderTown)" <> | Date | Tue, 7 Jun 2022 15:16:34 +0800 |
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On 2022/6/7 1:12, Ard Biesheuvel wrote: > On Mon, 6 Jun 2022 at 17:52, Ard Biesheuvel <ardb@kernel.org> wrote: >> >> Hello Zhen Lei, >> >> On Mon, 6 Jun 2022 at 14:49, Zhen Lei <thunder.leizhen@huawei.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!
Yes, it took a lot of time, a lot of boards.
>> >>> 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.
Right, NX should also be set. I will try MT_MEMORY_RO.
>> >> 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.
I'll keep the section mapping first, because the fix for adding the shareable attribute is explicit.
> . >
-- Regards, Zhen Lei
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