On 7 August 2015 at 11:37, Matt Fleming <[email protected]> wrote: > On Fri, 07 Aug, at 11:00:17AM, Ard Biesheuvel wrote: >> >> The EFI memory types are not exclusive, and so many regions will have >> all of the above set. The UEFI spec does not define how to interpret >> these superimposed attributes, it is up to the OS to decide on a >> consistent approach. >> >> For instance, this region (captured from a arm64 boot log with >> uefi_debug set on the command line) >> >> [Runtime Data |RUN| | | | |WB|WT|WC|UC] >> >> would be mapped uncached when following the above logic, while it >> makes more sense to map using PAGE_KERNEL in this case. > > Urgh... good point Ard. Right now this is limited to the GHES driver, so > it's unclear whether this patch is buggy in practice or not. > > Does it *ever* make sense to map a region as cacheable (WB/WT/WC) on > arm64 for the APEI/GHES case? Does the firmware handle the necessary > cache flushing? >
No it does not. Currently, we only consider EFI_MEMORY_WB when discovering system RAM from the UEFI memory map, so the direct linear mapping should have a hole where the APEI/GHES regions lives if it doesn't have the WB attribute set. This means we can map it WT/WC/UC without violating architectural rules regarding mismatches attributes, but it requires an explicit ioremap() >> From the spec: >> >> """ >> EFI_MEMORY_UC: The memory region supports being configured as not cacheable. >> EFI_MEMORY_WC: The memory region supports being configured as write >> combining. >> EFI_MEMORY_WT: The memory region supports being configured as >> cacheable with a “write through” policy. Writes that hit in the cache >> will also be written to main memory. >> EFI_MEMORY_WB: The memory region supports being configured as >> cacheable with a “write back” policy. Reads and writes that hit in the >> cache do not propagate to main memory. Dirty data is written back to >> main memory when a new cache line is allocated. >> """ > > Jonathan, can you please provide the EFI memory map region attributes > for the GHES region that requires this series? > > -- > Matt Fleming, Intel Open Source Technology Center -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
