diff options
Diffstat (limited to 'Documentation/x86')
| -rw-r--r-- | Documentation/x86/early-microcode.txt | 70 | ||||
| -rw-r--r-- | Documentation/x86/microcode.txt | 137 | ||||
| -rw-r--r-- | Documentation/x86/orc-unwinder.txt | 179 |
3 files changed, 316 insertions, 70 deletions
diff --git a/Documentation/x86/early-microcode.txt b/Documentation/x86/early-microcode.txt deleted file mode 100644 index 07749e7..0000000 --- a/Documentation/x86/early-microcode.txt +++ /dev/null @@ -1,70 +0,0 @@ -Early load microcode -==================== -By Fenghua Yu <fenghua.yu@intel.com> - -Kernel can update microcode in early phase of boot time. Loading microcode early -can fix CPU issues before they are observed during kernel boot time. - -Microcode is stored in an initrd file. The microcode is read from the initrd -file and loaded to CPUs during boot time. - -The format of the combined initrd image is microcode in cpio format followed by -the initrd image (maybe compressed). Kernel parses the combined initrd image -during boot time. The microcode file in cpio name space is: -on Intel: kernel/x86/microcode/GenuineIntel.bin -on AMD : kernel/x86/microcode/AuthenticAMD.bin - -During BSP boot (before SMP starts), if the kernel finds the microcode file in -the initrd file, it parses the microcode and saves matching microcode in memory. -If matching microcode is found, it will be uploaded in BSP and later on in all -APs. - -The cached microcode patch is applied when CPUs resume from a sleep state. - -There are two legacy user space interfaces to load microcode, either through -/dev/cpu/microcode or through /sys/devices/system/cpu/microcode/reload file -in sysfs. - -In addition to these two legacy methods, the early loading method described -here is the third method with which microcode can be uploaded to a system's -CPUs. - -The following example script shows how to generate a new combined initrd file in -/boot/initrd-3.5.0.ucode.img with original microcode microcode.bin and -original initrd image /boot/initrd-3.5.0.img. - -mkdir initrd -cd initrd -mkdir -p kernel/x86/microcode -cp ../microcode.bin kernel/x86/microcode/GenuineIntel.bin (or AuthenticAMD.bin) -find . | cpio -o -H newc >../ucode.cpio -cd .. -cat ucode.cpio /boot/initrd-3.5.0.img >/boot/initrd-3.5.0.ucode.img - -Builtin microcode -================= - -We can also load builtin microcode supplied through the regular firmware -builtin method CONFIG_FIRMWARE_IN_KERNEL. Only 64-bit is currently -supported. - -Here's an example: - -CONFIG_FIRMWARE_IN_KERNEL=y -CONFIG_EXTRA_FIRMWARE="intel-ucode/06-3a-09 amd-ucode/microcode_amd_fam15h.bin" -CONFIG_EXTRA_FIRMWARE_DIR="/lib/firmware" - -This basically means, you have the following tree structure locally: - -/lib/firmware/ -|-- amd-ucode -... -| |-- microcode_amd_fam15h.bin -... -|-- intel-ucode -... -| |-- 06-3a-09 -... - -so that the build system can find those files and integrate them into -the final kernel image. The early loader finds them and applies them. diff --git a/Documentation/x86/microcode.txt b/Documentation/x86/microcode.txt new file mode 100644 index 0000000..f57e1b4 --- /dev/null +++ b/Documentation/x86/microcode.txt @@ -0,0 +1,137 @@ + The Linux Microcode Loader + +Authors: Fenghua Yu <fenghua.yu@intel.com> + Borislav Petkov <bp@suse.de> + +The kernel has a x86 microcode loading facility which is supposed to +provide microcode loading methods in the OS. Potential use cases are +updating the microcode on platforms beyond the OEM End-Of-Life support, +and updating the microcode on long-running systems without rebooting. + +The loader supports three loading methods: + +1. Early load microcode +======================= + +The kernel can update microcode very early during boot. Loading +microcode early can fix CPU issues before they are observed during +kernel boot time. + +The microcode is stored in an initrd file. During boot, it is read from +it and loaded into the CPU cores. + +The format of the combined initrd image is microcode in (uncompressed) +cpio format followed by the (possibly compressed) initrd image. The +loader parses the combined initrd image during boot. + +The microcode files in cpio name space are: + +on Intel: kernel/x86/microcode/GenuineIntel.bin +on AMD : kernel/x86/microcode/AuthenticAMD.bin + +During BSP (BootStrapping Processor) boot (pre-SMP), the kernel +scans the microcode file in the initrd. If microcode matching the +CPU is found, it will be applied in the BSP and later on in all APs +(Application Processors). + +The loader also saves the matching microcode for the CPU in memory. +Thus, the cached microcode patch is applied when CPUs resume from a +sleep state. + +Here's a crude example how to prepare an initrd with microcode (this is +normally done automatically by the distribution, when recreating the +initrd, so you don't really have to do it yourself. It is documented +here for future reference only). + +--- + #!/bin/bash + + if [ -z "$1" ]; then + echo "You need to supply an initrd file" + exit 1 + fi + + INITRD="$1" + + DSTDIR=kernel/x86/microcode + TMPDIR=/tmp/initrd + + rm -rf $TMPDIR + + mkdir $TMPDIR + cd $TMPDIR + mkdir -p $DSTDIR + + if [ -d /lib/firmware/amd-ucode ]; then + cat /lib/firmware/amd-ucode/microcode_amd*.bin > $DSTDIR/AuthenticAMD.bin + fi + + if [ -d /lib/firmware/intel-ucode ]; then + cat /lib/firmware/intel-ucode/* > $DSTDIR/GenuineIntel.bin + fi + + find . | cpio -o -H newc >../ucode.cpio + cd .. + mv $INITRD $INITRD.orig + cat ucode.cpio $INITRD.orig > $INITRD + + rm -rf $TMPDIR +--- + +The system needs to have the microcode packages installed into +/lib/firmware or you need to fixup the paths above if yours are +somewhere else and/or you've downloaded them directly from the processor +vendor's site. + +2. Late loading +=============== + +There are two legacy user space interfaces to load microcode, either through +/dev/cpu/microcode or through /sys/devices/system/cpu/microcode/reload file +in sysfs. + +The /dev/cpu/microcode method is deprecated because it needs a special +userspace tool for that. + +The easier method is simply installing the microcode packages your distro +supplies and running: + +# echo 1 > /sys/devices/system/cpu/microcode/reload + +as root. + +The loading mechanism looks for microcode blobs in +/lib/firmware/{intel-ucode,amd-ucode}. The default distro installation +packages already put them there. + +3. Builtin microcode +==================== + +The loader supports also loading of a builtin microcode supplied through +the regular firmware builtin method CONFIG_FIRMWARE_IN_KERNEL. Only +64-bit is currently supported. + +Here's an example: + +CONFIG_FIRMWARE_IN_KERNEL=y +CONFIG_EXTRA_FIRMWARE="intel-ucode/06-3a-09 amd-ucode/microcode_amd_fam15h.bin" +CONFIG_EXTRA_FIRMWARE_DIR="/lib/firmware" + +This basically means, you have the following tree structure locally: + +/lib/firmware/ +|-- amd-ucode +... +| |-- microcode_amd_fam15h.bin +... +|-- intel-ucode +... +| |-- 06-3a-09 +... + +so that the build system can find those files and integrate them into +the final kernel image. The early loader finds them and applies them. + +Needless to say, this method is not the most flexible one because it +requires rebuilding the kernel each time updated microcode from the CPU +vendor is available. diff --git a/Documentation/x86/orc-unwinder.txt b/Documentation/x86/orc-unwinder.txt new file mode 100644 index 0000000..af0c9a4 --- /dev/null +++ b/Documentation/x86/orc-unwinder.txt @@ -0,0 +1,179 @@ +ORC unwinder +============ + +Overview +-------- + +The kernel CONFIG_ORC_UNWINDER option enables the ORC unwinder, which is +similar in concept to a DWARF unwinder. The difference is that the +format of the ORC data is much simpler than DWARF, which in turn allows +the ORC unwinder to be much simpler and faster. + +The ORC data consists of unwind tables which are generated by objtool. +They contain out-of-band data which is used by the in-kernel ORC +unwinder. Objtool generates the ORC data by first doing compile-time +stack metadata validation (CONFIG_STACK_VALIDATION). After analyzing +all the code paths of a .o file, it determines information about the +stack state at each instruction address in the file and outputs that +information to the .orc_unwind and .orc_unwind_ip sections. + +The per-object ORC sections are combined at link time and are sorted and +post-processed at boot time. The unwinder uses the resulting data to +correlate instruction addresses with their stack states at run time. + + +ORC vs frame pointers +--------------------- + +With frame pointers enabled, GCC adds instrumentation code to every +function in the kernel. The kernel's .text size increases by about +3.2%, resulting in a broad kernel-wide slowdown. Measurements by Mel +Gorman [1] have shown a slowdown of 5-10% for some workloads. + +In contrast, the ORC unwinder has no effect on text size or runtime +performance, because the debuginfo is out of band. So if you disable +frame pointers and enable the ORC unwinder, you get a nice performance +improvement across the board, and still have reliable stack traces. + +Ingo Molnar says: + + "Note that it's not just a performance improvement, but also an + instruction cache locality improvement: 3.2% .text savings almost + directly transform into a similarly sized reduction in cache + footprint. That can transform to even higher speedups for workloads + whose cache locality is borderline." + +Another benefit of ORC compared to frame pointers is that it can +reliably unwind across interrupts and exceptions. Frame pointer based +unwinds can sometimes skip the caller of the interrupted function, if it +was a leaf function or if the interrupt hit before the frame pointer was +saved. + +The main disadvantage of the ORC unwinder compared to frame pointers is +that it needs more memory to store the ORC unwind tables: roughly 2-4MB +depending on the kernel config. + + +ORC vs DWARF +------------ + +ORC debuginfo's advantage over DWARF itself is that it's much simpler. +It gets rid of the complex DWARF CFI state machine and also gets rid of +the tracking of unnecessary registers. This allows the unwinder to be +much simpler, meaning fewer bugs, which is especially important for +mission critical oops code. + +The simpler debuginfo format also enables the unwinder to be much faster +than DWARF, which is important for perf and lockdep. In a basic +performance test by Jiri Slaby [2], the ORC unwinder was about 20x +faster than an out-of-tree DWARF unwinder. (Note: That measurement was +taken before some performance tweaks were added, which doubled +performance, so the speedup over DWARF may be closer to 40x.) + +The ORC data format does have a few downsides compared to DWARF. ORC +unwind tables take up ~50% more RAM (+1.3MB on an x86 defconfig kernel) +than DWARF-based eh_frame tables. + +Another potential downside is that, as GCC evolves, it's conceivable +that the ORC data may end up being *too* simple to describe the state of +the stack for certain optimizations. But IMO this is unlikely because +GCC saves the frame pointer for any unusual stack adjustments it does, +so I suspect we'll really only ever need to keep track of the stack +pointer and the frame pointer between call frames. But even if we do +end up having to track all the registers DWARF tracks, at least we will +still be able to control the format, e.g. no complex state machines. + + +ORC unwind table generation +--------------------------- + +The ORC data is generated by objtool. With the existing compile-time +stack metadata validation feature, objtool already follows all code +paths, and so it already has all the information it needs to be able to +generate ORC data from scratch. So it's an easy step to go from stack +validation to ORC data generation. + +It should be possible to instead generate the ORC data with a simple +tool which converts DWARF to ORC data. However, such a solution would +be incomplete due to the kernel's extensive use of asm, inline asm, and +special sections like exception tables. + +That could be rectified by manually annotating those special code paths +using GNU assembler .cfi annotations in .S files, and homegrown +annotations for inline asm in .c files. But asm annotations were tried +in the past and were found to be unmaintainable. They were often +incorrect/incomplete and made the code harder to read and keep updated. +And based on looking at glibc code, annotating inline asm in .c files +might be even worse. + +Objtool still needs a few annotations, but only in code which does +unusual things to the stack like entry code. And even then, far fewer +annotations are needed than what DWARF would need, so they're much more +maintainable than DWARF CFI annotations. + +So the advantages of using objtool to generate ORC data are that it +gives more accurate debuginfo, with very few annotations. It also +insulates the kernel from toolchain bugs which can be very painful to +deal with in the kernel since we often have to workaround issues in +older versions of the toolchain for years. + +The downside is that the unwinder now becomes dependent on objtool's +ability to reverse engineer GCC code flow. If GCC optimizations become +too complicated for objtool to follow, the ORC data generation might +stop working or become incomplete. (It's worth noting that livepatch +already has such a dependency on objtool's ability to follow GCC code +flow.) + +If newer versions of GCC come up with some optimizations which break +objtool, we may need to revisit the current implementation. Some +possible solutions would be asking GCC to make the optimizations more +palatable, or having objtool use DWARF as an additional input, or +creating a GCC plugin to assist objtool with its analysis. But for now, +objtool follows GCC code quite well. + + +Unwinder implementation details +------------------------------- + +Objtool generates the ORC data by integrating with the compile-time +stack metadata validation feature, which is described in detail in +tools/objtool/Documentation/stack-validation.txt. After analyzing all +the code paths of a .o file, it creates an array of orc_entry structs, +and a parallel array of instruction addresses associated with those +structs, and writes them to the .orc_unwind and .orc_unwind_ip sections +respectively. + +The ORC data is split into the two arrays for performance reasons, to +make the searchable part of the data (.orc_unwind_ip) more compact. The +arrays are sorted in parallel at boot time. + +Performance is further improved by the use of a fast lookup table which +is created at runtime. The fast lookup table associates a given address +with a range of indices for the .orc_unwind table, so that only a small +subset of the table needs to be searched. + + +Etymology +--------- + +Orcs, fearsome creatures of medieval folklore, are the Dwarves' natural +enemies. Similarly, the ORC unwinder was created in opposition to the +complexity and slowness of DWARF. + +"Although Orcs rarely consider multiple solutions to a problem, they do +excel at getting things done because they are creatures of action, not +thought." [3] Similarly, unlike the esoteric DWARF unwinder, the +veracious ORC unwinder wastes no time or siloconic effort decoding +variable-length zero-extended unsigned-integer byte-coded +state-machine-based debug information entries. + +Similar to how Orcs frequently unravel the well-intentioned plans of +their adversaries, the ORC unwinder frequently unravels stacks with +brutal, unyielding efficiency. + +ORC stands for Oops Rewind Capability. + + +[1] https://lkml.kernel.org/r/20170602104048.jkkzssljsompjdwy@suse.de +[2] https://lkml.kernel.org/r/d2ca5435-6386-29b8-db87-7f227c2b713a@suse.cz +[3] http://dustin.wikidot.com/half-orcs-and-orcs |
