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132 lines
5.1 KiB
132 lines
5.1 KiB
#pragma once
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#include "arch/common.h"
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#define ARCH_SIG_JMP_OFF 8
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/**
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* Initializes a context, zeros out registers, and sets the Instruction and
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* Stack pointers
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* @param actx arch_context to init
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* @param ip value to set instruction pointer to
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* @param sp value to set stack pointer to
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*/
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static void __attribute__((noinline)) arch_context_init(struct arch_context *actx, reg_t ip, reg_t sp)
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{
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memset(&actx->mctx, 0, sizeof(mcontext_t));
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memset((void *)actx->regs, 0, sizeof(reg_t) * UREG_COUNT);
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if (sp) {
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/*
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* context_switch conventions: bp is expected to be on top of the stack
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* when co-op context switching..
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*
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* Temporarily switches the active stack to the stack pointer stored in sp
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* to push the stack pointer sp to the top of its own stack.
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* This acts as the base pointer
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*/
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asm volatile("movq %%rsp, %%rbx\n\t" /* Temporarily save pointer of active stack to B */
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"movq %%rax, %%rsp\n\t" /* Set active stack to stack pointer in A(C variable sp) */
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"pushq %%rax\n\t" /* Push A(C variable sp) onto the stack at sp */
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"movq %%rsp, %%rax\n\t" /* Write the incremented stack pointer to A(C variable sp) */
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"movq %%rbx, %%rsp\n\t" /* Restore original stack saved in B */
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: "=a"(sp)
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: "a"(sp)
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: "memory", "cc", "rbx");
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}
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actx->regs[UREG_RSP] = sp;
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actx->regs[UREG_RIP] = ip;
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actx->variant = ARCH_CONTEXT_QUICK;
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}
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/**
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* @param current - the registers and context of the thing running
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* @param next - the registers and context of what we're switching to
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* @return always returns 0, indicating success
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*
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* NULL in either of these values indicates the "no sandbox to execute" state,
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* which defaults to resuming execution of main
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*/
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static inline int
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arch_context_switch(struct arch_context *current, struct arch_context *next)
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{
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/* Assumption: Software Interrupts are disabled by caller */
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assert(software_interrupt_is_disabled);
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/* if both current and next are NULL, there is no state change */
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assert(current != NULL || next != NULL);
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/* Assumption: The caller does not switch to itself */
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assert(current != next);
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/* Set any NULLs to worker_thread_base_context to resume execution of main */
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if (current == NULL) current = &worker_thread_base_context;
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if (next == NULL) next = &worker_thread_base_context;
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/* Assumption: The context we are switching to should have saved a context in some form */
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assert(next->variant == ARCH_CONTEXT_QUICK || next->variant != ARCH_CONTEXT_UNUSED);
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reg_t *current_registers = current->regs, *next_registers = next->regs;
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assert(current_registers && next_registers);
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asm volatile(
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/* Create a new stack frame */
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"pushq %%rbp\n\t" /* stack[stack_len++] = base_pointer */
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"movq %%rsp, %%rbp\n\t" /* base_pointer = stack_pointer. Start new Frame */
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/*
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* Save the IP and stack pointer to the context of the sandbox we're switching from
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*/
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"movq $2f, 8(%%rax)\n\t" /* Write the address of label 2 to current_registers[1] (instruction_pointer). */
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"movq %%rsp, (%%rax)\n\t" /* current_registers[0] (stack_pointer) = stack_pointer */
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"movq $1, (%%rcx)\n\t" /* current->variant = ARCH_CONTEXT_QUICK; */
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/*
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* Check if the variant of the context we're trying to switch to is SLOW (mcontext-based)
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* If it is, jump to label 1 to restore the preempted sandbox
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*/
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"cmpq $2, (%%rcx)\n\t" /* if (next->variant == ARCH_CONTEXT_SLOW); */
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"je 1f\n\t" /* goto 1; restore the existing sandbox using mcontext */
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/*
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* Fast Path
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* We can just write update the stack pointer and jump to the target instruction
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*/
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"movq (%%rbx), %%rsp\n\t" /* stack_pointer = next_registers[0] (stack_pointer) */
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"jmpq *8(%%rbx)\n\t" /* immediate jump to next_registers[1] (instruction_pointer) */
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/*
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* Slow Path
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* If the variant is ARCH_CONTEXT_SLOW, that means the sandbox was preempted and we need to
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* fallback to a full mcontext-based context switch. We do this by invoking
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* arch_context_mcontext_restore, which fires a SIGUSR1 signal. The SIGUSR1 signal handler
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* executes the mcontext-based context switch.
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*/
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"1:\n\t"
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"call arch_context_mcontext_restore\n\t"
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".align 8\n\t"
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/*
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* This label was written to the instruction pointer of the sandbox that was switched away from
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* The sandbox either resumes at label 2 or 3 depending on if an offset of 8 is used.
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*/
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"2:\n\t"
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"movq $3, (%%rdx)\n\t" /* next->variant = ARCH_CONTEXT_QUICK; */
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".align 8\n\t"
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/* This label is used in conjunction with a static offset */
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"3:\n\t"
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/* TODO: Should we set next->variant = ARCH_CONTEXT_SLOW here?;*/
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"popq %%rbp\n\t" /* base_pointer = stack[--stack_len]; Base Pointer is restored */
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:
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: "a"(current_registers), "b"(next_registers), "c"(¤t->variant), "d"(&next->variant)
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: "memory", "cc", "rsi", "rdi", "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15", "xmm0", "xmm1", "xmm2",
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"xmm3", "xmm4", "xmm5", "xmm6", "xmm7", "xmm8", "xmm9", "xmm10", "xmm11", "xmm12", "xmm13", "xmm14",
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"xmm15");
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worker_thread_is_switching_context = false;
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return 0;
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}
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