On 15.09.20 16:16, Ard Biesheuvel wrote: > On Tue, 15 Sep 2020 at 17:05, Grant Likely <grant.lik...@arm.com> wrote: >> >> >> >> On 15/09/2020 14:46, Leif Lindholm wrote: >>> On Tue, Sep 15, 2020 at 14:14:30 +0100, Grant Likely wrote: >>>>> The EFI stub in Linux removes /memreserve/ entries from the DT before >>>>> handing it to the kernel proper. >>>>> >>>>> commit 0ceac9e094b065fe3fec19669740f338d3480498 >>>>> Author: Mark Salter <msal...@redhat.com> >>>>> Date: Mon Sep 8 13:01:08 2014 -0400 >>>>> >>>>> efi/arm64: Fix fdt-related memory reservation >>>> >>>> Does that still make sense? I understand why it was done, but is it >>>> right to ignore those reservations outright? >>> >>> Yes. It is duplication of (sources of) information, forcing the >>> operating system to make runtime, or compile time, judgement calls of >>> which source(s) of information to respect. >> >> Not quite that simple. We're not talking about a clean cut-over from >> non-UEFI to UEFI platforms, but rather a phased transition where with a >> given DT, both the non-UEFI and UEFI boot paths need to work. e.g., >> U-Boot platforms where most people are using 'bootm', but want to start >> encouraging them to use the UEFI infrastructure. >> >> Or in other words, the master source of information is the .dts file, >> not the firmware itself. >> >> The other issue is that the reserved memory region may not be about >> firmware at all, but rather a memory layout that is wanted only by the >> OS. Regardless of the approach we take here, those regions must be >> respected. >> >>>> As more U-Boot platforms >>>> turn on UEFI there could be unexpected consequences if the memory >>>> reservation block are silently ignored. I'm think that on the U-Boot >>>> platforms it is more likely that /memreserve/ is in use. >>> >>> That should also make it easy to intercept? Like putting a hook in the >>> DT update code that triggers build error/warning (or even update the >>> UEFI memory map) if someone is trying to memreserve with the UEFI >>> interface enabled. >> >> It should not be an error to use /memreserve/. That creates a hard break >> between UEFI and non-UEFI boot paths for /memreserve/. Updating the >> memory map is fine, which leads to the question of what memory type >> should be used? >> >> EfiBootServicesData: Memory still gets mapped in the linear map, but >> nothing protects it after ExitBootServices (would require leaving >> /memreserve/ intact so the OS knows to protect them). >> >> EfiReservedMemory: (As I understand it) Doesn't need /memreserve/, but >> causes a change in behaviour. The memory will not appear in the linear >> map. This will possibly cause problems with existing drivers >> > > I wouldn't expect so. Unlike /reserved-memory nodes, which can be > referenced by other nodes and explicitly tagged as reusable, > /memreserve/s are anonymous holes that are punched into the memory > map, so I don't see how a driver would be able to get a reference to > that memory (and gets its linear address if it _happens_ to be in > lowmem in the first place.) > >> EfiRuntimeServicesData: Keeps the region protected and in the linear >> map, but feels 'wrong'. An OS might decide to reclaim it anyway if it >> doesn't use runtime services (against spec?). >> > > EfiRuntimeServicesData is not covered by the linear map, but will map > it in the EFI page tables for access by the firmware itself at > runtime, so it is not an option here.
The device-tree spec specifically says /mmemreserve/ serve to "define an entry for the devicetree blob’s memory reservation table." If I look at Linux code I find things like: /* First 4KB has trampoline code for secondary cores. */ /memreserve/ 0x00000000 0x0001000; /* firmware-provided startup stubs live here, where the secondary CPUs are * spinning. */ /memreserve/ 0x00000000 0x00001000; /memreserve/ 0x10000000 0x0004000;*/ /* DSP RAM */ CPUs with a 'spin-table' enable-method "should spin outside of the kernel in a reserved area of memory (communicated to the kernel by a /memreserve/ region in the device tree". This usage is not what I would have understood under the description in the devicetree spec. So the DT spec needs some clarification. The problem with EfiRuntimeServicesData is that it can be moved anywhere in the virtual address space via SetVirtualAddressMap(). But do we know that the firmware can handle such a relocation? Best regards Heinrich > > >> >>> >>>> It should be fine for /memreserve/ entries to get applied to the memmap >>>> during boot. Are there problems that I'm missing? >>> >>> Sure. They can be applied in the UEFI memory map. By u-boot, during >>> boot. >>> >>> / >>> Leif >>> >>>> g. >>>> IMPORTANT NOTICE: The contents of this email and any attachments are >>>> confidential and may also be privileged. If you are not the intended >>>> recipient, please notify the sender immediately and do not disclose the >>>> contents to any other person, use it for any purpose, or store or copy the >>>> information in any medium. Thank you. >>>> _______________________________________________ >>>> boot-architecture mailing list >>>> boot-architecture@lists.linaro.org >>>> https://lists.linaro.org/mailman/listinfo/boot-architecture >> IMPORTANT NOTICE: The contents of this email and any attachments are >> confidential and may also be privileged. If you are not the intended >> recipient, please notify the sender immediately and do not disclose the >> contents to any other person, use it for any purpose, or store or copy the >> information in any medium. Thank you. > _______________________________________________ > boot-architecture mailing list > boot-architecture@lists.linaro.org > https://lists.linaro.org/mailman/listinfo/boot-architecture > _______________________________________________ boot-architecture mailing list boot-architecture@lists.linaro.org https://lists.linaro.org/mailman/listinfo/boot-architecture
