Gives an introduction and overview to the Hexagon target Signed-off-by: Taylor Simpson <tsimp...@quicinc.com> --- target/hexagon/README | 254 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 254 insertions(+) create mode 100644 target/hexagon/README
diff --git a/target/hexagon/README b/target/hexagon/README new file mode 100644 index 0000000..3b65688 --- /dev/null +++ b/target/hexagon/README @@ -0,0 +1,254 @@ +Hexagon is Qualcomm's very long instruction word (VLIW) digital signal +processor(DSP). + +The following versions of the Hexagon core are supported + Scalar core: v67 + https://developer.qualcomm.com/downloads/qualcomm-hexagon-v67-programmer-s-reference-manual + +We presented an overview of the project at the 2019 KVM Forum. + https://kvmforum2019.sched.com/event/Tmwc/qemu-hexagon-automatic-translation-of-the-isa-manual-pseudcode-to-tiny-code-instructions-of-a-vliw-architecture-niccolo-izzo-revng-taylor-simpson-qualcomm-innovation-center + +*** Tour of the code *** + +The qemu-hexagon implementation is a combination of qemu and the Hexagon +architecture library (aka archlib). The three primary directories with +Hexagon-specific code are + + qemu/target/hexagon + This has all the instruction and packet semantics + qemu/target/hexagon/imported + These files are imported with very little modification from archlib + *.idef Instruction semantics definition + macros.def Mapping of macros to instruction attributes + encode*.def Encoding patterns for each instruction + iclass.def Instruction class definitions used to determine + legal VLIW slots for each instruction + qemu/linux-user/hexagon + Helpers for loading the ELF file and making Linux system calls, + signals, etc + +We start with scripts that generate a bunch of include files. This +is a two step process. The first step is to use the C preprocessor to expand +macros inside the architecture definition files. This is done in +target/hexagon/gen_semantics.c. This step produces + <BUILD_DIR>/hexagon-linux-user/semantics_generated.pyinc. +That file is consumed by the following python scripts to produce the indicated +header files in <BUILD_DIR>/hexagon-linux-user + gen_shortcode.py -> shortcode_generated.h + gen_helper_protos.py -> helper_protos_generated.h + gen_tcg_funcs.py -> tcg_funcs_generated.h + gen_helper_funcs.py -> helper_funcs_generated.h + +Qemu helper functions have 3 parts + DEF_HELPER declaration indicates the signature of the helper + gen_helper_<NAME> will generate a TCG call to the helper function + The helper implementation + +Here's an example of the A2_add instruction. + Instruction tag A2_add + Assembly syntax "Rd32=add(Rs32,Rt32)" + Instruction semantics "{ RdV=RsV+RtV;}" + +By convention, the operands are identified by letter + RdV is the destination register + RsV, RtV are source registers + +The generator uses the operand naming conventions (see large comment in +hex_common.py) to determine the signature of the helper function. Here are the +results for A2_add + +helper_protos_generated.h + #ifndef fGEN_TCG_A2_add + DEF_HELPER_3(A2_add, s32, env, s32, s32) + #endif + +tcg_funcs_generated.h + DEF_TCG_FUNC(A2_add, /* { RdV=RsV+RtV;} */ + { + /* A2_add */ + DECL_RREG_d(RdV, RdN, 0, 0); + DECL_RREG_s(RsV, RsN, 1, 0); + DECL_RREG_t(RtV, RtN, 2, 0); + READ_RREG_s(RsV, RsN); + READ_RREG_t(RtV, RtN); + #ifdef fGEN_TCG_A2_add + fGEN_TCG_A2_add({ RdV=RsV+RtV;}); + #else + do { + gen_helper_A2_add(RdV, cpu_env, RsV, RtV); + } while (0); + #endif + WRITE_RREG_d(RdN, RdV); + FREE_RREG_d(RdV); + FREE_RREG_s(RsV); + FREE_RREG_t(RtV); + /* A2_add */ + }) + +helper_funcs_generated.h + #ifndef fGEN_TCG_A2_add + int32_t HELPER(A2_add)(CPUHexagonState *env, int32_t RsV, int32_t RtV) + { + uint32_t slot __attribute__((unused)) = 4; + int32_t RdV = 0; + { RdV=RsV+RtV;} + return RdV; + } + #endif + +For each operand, there are macros for DECL, FREE, READ, WRITE. These are +defined in macros.h. Note that we append the operand type to the macro name, +which allows us to specialize the TCG code tenerated. For read-only operands, +DECL simply declares the TCGv variable (no need for tcg_temp_local_new()), +and READ will assign from the TCGv corresponding to the GPR, and FREE doesn't +have to do anything. Also, note that the WRITE macros update the disassembly +context to be processed when the packet commits (see "Packet Semantics" below). + +Note the fGEN_TCG_A2_add macro. This macro allows us to generate TCG code +instead of a call to the helper. If defined, the macro takes 1 argument. + C semantics (aka short code) + +This allows the code generator to override the auto-generated code. In some +cases this is necessary for correct execution. We can also override for +faster emulation. For example, calling a helper for add is more expensive +than generating a TCG add operation. + +The gen_tcg.h file has any overrides. For example, + #define fGEN_TCG_A2_add(GENHLPR, SHORTCODE) \ + tcg_gen_add_tl(RdV, RsV, RtV) + +The gen_tcg.h file is included twice +1) In genptr.c, it overrides the semantics from tcg_funcs_generated.h +2) In helper.h, it prevents the generation of helpers for overridden + instructions. Notice the #ifndef fGEN_TCG_A2_add above in both + helper_protos_generated.h and helper_functions.h + +The instruction semantics C code relies heavily on macros. In cases where the +C semantics are specified only with macros, we can override the default with +the short semantics option and #define the macros to generate TCG code. One +example is L2_loadw_locked: + Instruction tag L2_loadw_locked + Assembly syntax "Rd32=memw_locked(Rs32)" + Instruction semantics "{ fEA_REG(RsV); fLOAD_LOCKED(1,4,u,EA,RdV) }" + +In gen_tcg.h, we use the shortcode +#define fGEN_TCG_L2_loadw_locked(SHORTCODE) \ + SHORTCODE + +There are also cases where we brute force the TCG code generation. +Instructions with multiple definitions are examples. These require special +handling because qemu helpers can only return a single value. + +In addition to instruction semantics, we use a generator to create the decode +tree. This generation is also a two step process. The first step is to run +target/hexagon/gen_dectree_import.c to produce + <BUILD_DIR>/hexagon-linux-user/iset.py +This file is imported by target/hexagon/dectree.py to produce + <BUILD_DIR>/hexagon-linux-user/dectree_generated.h + +*** Key Files *** + +cpu.h + +This file contains the definition of the CPUHexagonState struct. It is the +runtime information for each thread and contains stuff like the GPR and +predicate registers. + +macros.h + +The Hexagon arch lib relies heavily on macros for the instruction semantics. +This is a great advantage for qemu because we can override them for different +purposes. You will also notice there are sometimes two definitions of a macro. +The QEMU_GENERATE variable determines whether we want the macro to generate TCG +code. If QEMU_GENERATE is not defined, we want the macro to generate vanilla +C code that will work in the helper implementation. + +translate.c + +The functions in this file generate TCG code for a translation block. Some +important functions in this file are + + gen_start_packet - initialize the data structures for packet semantics + gen_commit_packet - commit the register writes, stores, etc for a packet + decode_packet - disassemble a packet and generate code + +genptr.c +genptr_helpers.h +gen_tcg.h + +These files create a function for each instruction. It is mostly composed of +fGEN_TCG_<tag> definitions followed by including qemu_def_generated.h. The +genptr_helpers.h file contains helper functions that are invoked by the macros +in gen_tcg.h and macros.h + +op_helper.c + +This file contains the implementations of all the helpers. There are a few +general purpose helpers, but most of them are generated by including +qemu_def_generated.h. There are also several helpers used for debugging. + + +*** Packet Semantics *** + +VLIW packet semantics differ from serial semantics in that all input operands +are read, then the operations are performed, then all the results are written. +For exmaple, this packet performs a swap of registers r0 and r1 + { r0 = r1; r1 = r0 } +Note that the result is different if the instructions are executed serially. + +Packet semantics dictate that we defer any changes of state until the entire +packet is committed. We record the results of each instruction in a side data +structure, and update the visible processor state when we commit the packet. + +The data structures are divided between the runtime state and the translation +context. + +During the TCG generation (see translate.[ch]), we use the DisasContext to +track what needs to be done during packet commit. Here are the relevant +fields + + reg_log list of registers written + reg_log_idx index into ctx_reg_log + pred_log list of predicates written + pred_log_idx index into ctx_pred_log + store_width width of stores (indexed by slot) + +During runtime, the following fields in CPUHexagonState (see cpu.h) are used + + new_value new value of a given register + reg_written boolean indicating if register was written + new_pred_value new value of a predicate register + pred_written boolean indicating if predicate was written + mem_log_stores record of the stores (indexed by slot) + +*** Debugging *** + +You can turn on a lot of debugging by changing the HEX_DEBUG macro to 1 in +internal.h. This will stream a lot of information as it generates TCG and +executes the code. + +To track down nasty issues with Hexagon->TCG generation, we compare the +execution results with actual hardware running on a Hexagon Linux target. +Run qemu with the "-d cpu" option. Then, we can diff the results and figure +out where qemu and hardware behave differently. + +The stacks are located at different locations. We handle this by changing +env->stack_adjust in translate.c. First, set this to zero and run qemu. +Then, change env->stack_adjust to the difference between the two stack +locations. Then rebuild qemu and run again. That will produce a very +clean diff. + +Here are some handy places to set breakpoints + + At the call to gen_start_packet for a given PC (note that the line number + might change in the future) + br translate.c:602 if ctx->base.pc_next == 0xdeadbeef + The helper function for each instruction is named helper_<TAG>, so here's + an example that will set a breakpoint at the start + br helper_A2_add + If you have the HEX_DEBUG macro set, the following will be useful + At the start of execution of a packet for a given PC + br helper_debug_start_packet if env->gpr[41] == 0xdeadbeef + At the end of execution of a packet for a given PC + br helper_debug_commit_end if env->this_PC == 0xdeadbeef + -- 2.7.4
