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[92.145.124.62]) by smtp.gmail.com with ESMTPSA id a25-20020a5d4579000000b003048477729asm7566182wrc.81.2023.06.26.07.37.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 26 Jun 2023 07:37:29 -0700 (PDT) From: Alexandre Ghiti To: Jonathan Corbet , Paul Walmsley , Palmer Dabbelt , Albert Ou , =?utf-8?b?QmrDtnJuIFTDtnBlbA==?= , Andrew Jones , Conor Dooley , Sunil V L , Song Shuai , linux-doc@vger.kernel.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Cc: Alexandre Ghiti , Palmer Dabbelt , Atish Patra Subject: [PATCH v4 2/3] Documentation: riscv: Add early boot document Date: Mon, 26 Jun 2023 16:36:25 +0200 Message-Id: <20230626143626.106584-2-alexghiti@rivosinc.com> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230626143626.106584-1-alexghiti@rivosinc.com> References: <20230626143626.106584-1-alexghiti@rivosinc.com> MIME-Version: 1.0 X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1769777814593161849?= X-GMAIL-MSGID: =?utf-8?q?1769777814593161849?= This document describes the constraints and requirements of the early boot process in a RISC-V kernel. Signed-off-by: Alexandre Ghiti Reviewed-by: Björn Töpel Reviewed-by: Conor Dooley Reviewed-by: Sunil V L Reviewed-by: Andrew Jones Reviewed-by: Palmer Dabbelt Reviewed-by: Atish Patra Reviewed-by: Song Shuai Acked-by: Palmer Dabbelt Acked-by: Palmer Dabbelt --- Documentation/riscv/boot-image-header.rst | 3 - Documentation/riscv/boot.rst | 169 ++++++++++++++++++++++ Documentation/riscv/index.rst | 1 + 3 files changed, 170 insertions(+), 3 deletions(-) create mode 100644 Documentation/riscv/boot.rst diff --git a/Documentation/riscv/boot-image-header.rst b/Documentation/riscv/boot-image-header.rst index d7752533865f..a4a45310c4c4 100644 --- a/Documentation/riscv/boot-image-header.rst +++ b/Documentation/riscv/boot-image-header.rst @@ -7,9 +7,6 @@ Boot image header in RISC-V Linux This document only describes the boot image header details for RISC-V Linux. -TODO: - Write a complete booting guide. - The following 64-byte header is present in decompressed Linux kernel image:: u32 code0; /* Executable code */ diff --git a/Documentation/riscv/boot.rst b/Documentation/riscv/boot.rst new file mode 100644 index 000000000000..f890ac442c91 --- /dev/null +++ b/Documentation/riscv/boot.rst @@ -0,0 +1,169 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=============================================== +RISC-V Kernel Boot Requirements and Constraints +=============================================== + +:Author: Alexandre Ghiti +:Date: 23 May 2023 + +This document describes what the RISC-V kernel expects from bootloaders and +firmware, but also the constraints that any developer must have in mind when +touching the early boot process. For the purposes of this document, the +``early boot process`` refers to any code that runs before the final virtual +mapping is set up. + +Pre-kernel Requirements and Constraints +======================================= + +The RISC-V kernel expects the following of bootloaders and platform firmware: + +Register state +-------------- + +The RISC-V kernel expects: + + * ``$a0`` to contain the hartid of the current core. + * ``$a1`` to contain the address of the devicetree in memory. + +CSR state +--------- + +The RISC-V kernel expects: + + * ``$satp = 0``: the MMU, if present, must be disabled. + +Reserved memory for resident firmware +------------------------------------- + +The RISC-V kernel must not map any resident memory, or memory protected with +PMPs, in the direct mapping, so the firmware must correctly mark those regions +as per the devicetree specification and/or the UEFI specification. + +Kernel location +--------------- + +The RISC-V kernel expects to be placed at a PMD boundary (2MB aligned for rv64 +and 4MB aligned for rv32). Note that the EFI stub will physically relocate the +kernel if that's not the case. + +Hardware description +-------------------- + +The firmware can pass either a devicetree or ACPI tables to the RISC-V kernel. + +The devicetree is either passed directly to the kernel from the previous stage +using the ``$a1`` register, or when booting with UEFI, it can be passed using the +EFI configuration table. + +The ACPI tables are passed to the kernel using the EFI configuration table. In +this case, a tiny devicetree is still created by the EFI stub. Please refer to +"EFI stub and devicetree" section below for details about this devicetree. + +Kernel entrance +--------------- + +On SMP systems, there are 2 methods to enter the kernel: + +- ``RISCV_BOOT_SPINWAIT``: the firmware releases all harts in the kernel, one hart + wins a lottery and executes the early boot code while the other harts are + parked waiting for the initialization to finish. This method is mostly used to + support older firmwares without SBI HSM extension and M-mode RISC-V kernel. +- ``Ordered booting``: the firmware releases only one hart that will execute the + initialization phase and then will start all other harts using the SBI HSM + extension. The ordered booting method is the preferred booting method for + booting the RISC-V kernel because it can support cpu hotplug and kexec. + +UEFI +---- + +UEFI memory map +~~~~~~~~~~~~~~~ + +When booting with UEFI, the RISC-V kernel will use only the EFI memory map to +populate the system memory. + +The UEFI firmware must parse the subnodes of the ``/reserved-memory`` devicetree +node and abide by the devicetree specification to convert the attributes of +those subnodes (``no-map`` and ``reusable``) into their correct EFI equivalent +(refer to section "3.5.4 /reserved-memory and UEFI" of the devicetree +specification v0.4-rc1). + +RISCV_EFI_BOOT_PROTOCOL +~~~~~~~~~~~~~~~~~~~~~~~ + +When booting with UEFI, the EFI stub requires the boot hartid in order to pass +it to the RISC-V kernel in ``$a1``. The EFI stub retrieves the boot hartid using +one of the following methods: + +- ``RISCV_EFI_BOOT_PROTOCOL`` (**preferred**). +- ``boot-hartid`` devicetree subnode (**deprecated**). + +Any new firmware must implement ``RISCV_EFI_BOOT_PROTOCOL`` as the devicetree +based approach is deprecated now. + +Early Boot Requirements and Constraints +======================================= + +The RISC-V kernel's early boot process operates under the following constraints: + +EFI stub and devicetree +----------------------- + +When booting with UEFI, the devicetree is supplemented (or created) by the EFI +stub with the same parameters as arm64 which are described at the paragraph +"UEFI kernel support on ARM" in Documentation/arm/uefi.rst. + +Virtual mapping installation +---------------------------- + +The installation of the virtual mapping is done in 2 steps in the RISC-V kernel: + +1. ``setup_vm()`` installs a temporary kernel mapping in ``early_pg_dir`` which + allows discovery of the system memory. Only the kernel text/data are mapped + at this point. When establishing this mapping, no allocation can be done + (since the system memory is not known yet), so ``early_pg_dir`` page table is + statically allocated (using only one table for each level). + +2. ``setup_vm_final()`` creates the final kernel mapping in ``swapper_pg_dir`` + and takes advantage of the discovered system memory to create the linear + mapping. When establishing this mapping, the kernel can allocate memory but + cannot access it directly (since the direct mapping is not present yet), so + it uses temporary mappings in the fixmap region to be able to access the + newly allocated page table levels. + +For ``virt_to_phys()`` and ``phys_to_virt()`` to be able to correctly convert +direct mapping addresses to physical addresses, they need to know the start of +the DRAM. This happens after step 1, right before step 2 installs the direct +mapping (see ``setup_bootmem()`` function in arch/riscv/mm/init.c). Any usage of +those macros before the final virtual mapping is installed must be carefully +examined. + +Devicetree mapping via fixmap +----------------------------- + +As the ``reserved_mem`` array is initialized with virtual addresses established +by ``setup_vm()``, and used with the mapping established by +``setup_vm_final()``, the RISC-V kernel uses the fixmap region to map the +devicetree. This ensures that the devicetree remains accessible by both virtual +mappings. + +Pre-MMU execution +----------------- + +A few pieces of code need to run before even the first virtual mapping is +established. These are the installation of the first virtual mapping itself, +patching of early alternatives and the early parsing of the kernel command line. +That code must be very carefully compiled as: + +- ``-fno-pie``: This is needed for relocatable kernels which use ``-fPIE``, + since otherwise, any access to a global symbol would go through the GOT which + is only relocated virtually. +- ``-mcmodel=medany``: Any access to a global symbol must be PC-relative to + avoid any relocations to happen before the MMU is setup. +- *all* instrumentation must also be disabled (that includes KASAN, ftrace and + others). + +As using a symbol from a different compilation unit requires this unit to be +compiled with those flags, we advise, as much as possible, not to use external +symbols. diff --git a/Documentation/riscv/index.rst b/Documentation/riscv/index.rst index 175a91db0200..1f66062def6d 100644 --- a/Documentation/riscv/index.rst +++ b/Documentation/riscv/index.rst @@ -5,6 +5,7 @@ RISC-V architecture .. toctree:: :maxdepth: 1 + boot boot-image-header vm-layout hwprobe