236 lines
10 KiB
Plaintext
236 lines
10 KiB
Plaintext
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Hexagon is Qualcomm's very long instruction word (VLIW) digital signal
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processor(DSP).
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The following versions of the Hexagon core are supported
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Scalar core: v67
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https://developer.qualcomm.com/downloads/qualcomm-hexagon-v67-programmer-s-reference-manual
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We presented an overview of the project at the 2019 KVM Forum.
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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
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*** Tour of the code ***
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The qemu-hexagon implementation is a combination of qemu and the Hexagon
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architecture library (aka archlib). The three primary directories with
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Hexagon-specific code are
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qemu/target/hexagon
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This has all the instruction and packet semantics
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qemu/target/hexagon/imported
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These files are imported with very little modification from archlib
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*.idef Instruction semantics definition
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macros.def Mapping of macros to instruction attributes
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encode*.def Encoding patterns for each instruction
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iclass.def Instruction class definitions used to determine
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legal VLIW slots for each instruction
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qemu/linux-user/hexagon
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Helpers for loading the ELF file and making Linux system calls,
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signals, etc
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We start with scripts that generate a bunch of include files. This
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is a two step process. The first step is to use the C preprocessor to expand
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macros inside the architecture definition files. This is done in
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target/hexagon/gen_semantics.c. This step produces
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<BUILD_DIR>/target/hexagon/semantics_generated.pyinc.
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That file is consumed by the following python scripts to produce the indicated
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header files in <BUILD_DIR>/target/hexagon
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gen_opcodes_def.py -> opcodes_def_generated.h.inc
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gen_op_regs.py -> op_regs_generated.h.inc
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gen_printinsn.py -> printinsn_generated.h.inc
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gen_op_attribs.py -> op_attribs_generated.h.inc
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gen_helper_protos.py -> helper_protos_generated.h.inc
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gen_shortcode.py -> shortcode_generated.h.inc
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gen_tcg_funcs.py -> tcg_funcs_generated.c.inc
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gen_tcg_func_table.py -> tcg_func_table_generated.c.inc
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gen_helper_funcs.py -> helper_funcs_generated.c.inc
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Qemu helper functions have 3 parts
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DEF_HELPER declaration indicates the signature of the helper
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gen_helper_<NAME> will generate a TCG call to the helper function
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The helper implementation
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Here's an example of the A2_add instruction.
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Instruction tag A2_add
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Assembly syntax "Rd32=add(Rs32,Rt32)"
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Instruction semantics "{ RdV=RsV+RtV;}"
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By convention, the operands are identified by letter
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RdV is the destination register
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RsV, RtV are source registers
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The generator uses the operand naming conventions (see large comment in
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hex_common.py) to determine the signature of the helper function. Here are the
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results for A2_add
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helper_protos_generated.h.inc
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DEF_HELPER_3(A2_add, s32, env, s32, s32)
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tcg_funcs_generated.c.inc
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static void generate_A2_add(
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CPUHexagonState *env,
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DisasContext *ctx,
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Insn *insn,
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Packet *pkt)
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{
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TCGv RdV = tcg_temp_local_new();
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const int RdN = insn->regno[0];
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TCGv RsV = hex_gpr[insn->regno[1]];
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TCGv RtV = hex_gpr[insn->regno[2]];
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gen_helper_A2_add(RdV, cpu_env, RsV, RtV);
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gen_log_reg_write(RdN, RdV);
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ctx_log_reg_write(ctx, RdN);
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tcg_temp_free(RdV);
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}
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helper_funcs_generated.c.inc
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int32_t HELPER(A2_add)(CPUHexagonState *env, int32_t RsV, int32_t RtV)
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{
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uint32_t slot __attribute__((unused)) = 4;
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int32_t RdV = 0;
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{ RdV=RsV+RtV;}
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return RdV;
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}
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Note that generate_A2_add updates the disassembly context to be processed
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when the packet commits (see "Packet Semantics" below).
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The generator checks for fGEN_TCG_<tag> macro. This allows us to generate
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TCG code instead of a call to the helper. If defined, the macro takes 1
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argument.
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C semantics (aka short code)
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This allows the code generator to override the auto-generated code. In some
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cases this is necessary for correct execution. We can also override for
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faster emulation. For example, calling a helper for add is more expensive
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than generating a TCG add operation.
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The gen_tcg.h file has any overrides. For example, we could write
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#define fGEN_TCG_A2_add(GENHLPR, SHORTCODE) \
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tcg_gen_add_tl(RdV, RsV, RtV)
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The instruction semantics C code relies heavily on macros. In cases where the
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C semantics are specified only with macros, we can override the default with
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the short semantics option and #define the macros to generate TCG code. One
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example is L2_loadw_locked:
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Instruction tag L2_loadw_locked
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Assembly syntax "Rd32=memw_locked(Rs32)"
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Instruction semantics "{ fEA_REG(RsV); fLOAD_LOCKED(1,4,u,EA,RdV) }"
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In gen_tcg.h, we use the shortcode
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#define fGEN_TCG_L2_loadw_locked(SHORTCODE) \
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SHORTCODE
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There are also cases where we brute force the TCG code generation.
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Instructions with multiple definitions are examples. These require special
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handling because qemu helpers can only return a single value.
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In addition to instruction semantics, we use a generator to create the decode
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tree. This generation is also a two step process. The first step is to run
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target/hexagon/gen_dectree_import.c to produce
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<BUILD_DIR>/target/hexagon/iset.py
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This file is imported by target/hexagon/dectree.py to produce
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<BUILD_DIR>/target/hexagon/dectree_generated.h.inc
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*** Key Files ***
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cpu.h
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This file contains the definition of the CPUHexagonState struct. It is the
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runtime information for each thread and contains stuff like the GPR and
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predicate registers.
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macros.h
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The Hexagon arch lib relies heavily on macros for the instruction semantics.
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This is a great advantage for qemu because we can override them for different
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purposes. You will also notice there are sometimes two definitions of a macro.
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The QEMU_GENERATE variable determines whether we want the macro to generate TCG
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code. If QEMU_GENERATE is not defined, we want the macro to generate vanilla
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C code that will work in the helper implementation.
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translate.c
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The functions in this file generate TCG code for a translation block. Some
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important functions in this file are
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gen_start_packet - initialize the data structures for packet semantics
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gen_commit_packet - commit the register writes, stores, etc for a packet
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decode_and_translate_packet - disassemble a packet and generate code
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genptr.c
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gen_tcg.h
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These files create a function for each instruction. It is mostly composed of
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fGEN_TCG_<tag> definitions followed by including tcg_funcs_generated.c.inc.
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op_helper.c
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This file contains the implementations of all the helpers. There are a few
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general purpose helpers, but most of them are generated by including
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helper_funcs_generated.c.inc. There are also several helpers used for debugging.
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*** Packet Semantics ***
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VLIW packet semantics differ from serial semantics in that all input operands
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are read, then the operations are performed, then all the results are written.
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For exmaple, this packet performs a swap of registers r0 and r1
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{ r0 = r1; r1 = r0 }
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Note that the result is different if the instructions are executed serially.
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Packet semantics dictate that we defer any changes of state until the entire
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packet is committed. We record the results of each instruction in a side data
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structure, and update the visible processor state when we commit the packet.
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The data structures are divided between the runtime state and the translation
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context.
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During the TCG generation (see translate.[ch]), we use the DisasContext to
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track what needs to be done during packet commit. Here are the relevant
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fields
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reg_log list of registers written
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reg_log_idx index into ctx_reg_log
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pred_log list of predicates written
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pred_log_idx index into ctx_pred_log
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store_width width of stores (indexed by slot)
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During runtime, the following fields in CPUHexagonState (see cpu.h) are used
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new_value new value of a given register
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reg_written boolean indicating if register was written
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new_pred_value new value of a predicate register
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pred_written boolean indicating if predicate was written
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mem_log_stores record of the stores (indexed by slot)
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*** Debugging ***
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You can turn on a lot of debugging by changing the HEX_DEBUG macro to 1 in
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internal.h. This will stream a lot of information as it generates TCG and
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executes the code.
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To track down nasty issues with Hexagon->TCG generation, we compare the
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execution results with actual hardware running on a Hexagon Linux target.
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Run qemu with the "-d cpu" option. Then, we can diff the results and figure
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out where qemu and hardware behave differently.
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The stacks are located at different locations. We handle this by changing
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env->stack_adjust in translate.c. First, set this to zero and run qemu.
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Then, change env->stack_adjust to the difference between the two stack
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locations. Then rebuild qemu and run again. That will produce a very
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clean diff.
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Here are some handy places to set breakpoints
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At the call to gen_start_packet for a given PC (note that the line number
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might change in the future)
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br translate.c:602 if ctx->base.pc_next == 0xdeadbeef
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The helper function for each instruction is named helper_<TAG>, so here's
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an example that will set a breakpoint at the start
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br helper_A2_add
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If you have the HEX_DEBUG macro set, the following will be useful
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At the start of execution of a packet for a given PC
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br helper_debug_start_packet if env->gpr[41] == 0xdeadbeef
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At the end of execution of a packet for a given PC
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br helper_debug_commit_end if env->this_PC == 0xdeadbeef
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