zircon/kernel/arch/x86/thread.cpp

// Copyright 2016 The Fuchsia Authors
// Copyright (c) 2009 Corey Tabaka
// Copyright (c) 2014 Travis Geiselbrecht
// Copyright (c) 2015 Intel Corporation
//
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE file or at
// https://opensource.org/licenses/MIT

#include <arch/x86.h>
#include <arch/x86/descriptor.h>
#include <arch/x86/feature.h>
#include <arch/x86/mp.h>
#include <arch/x86/registers.h>
#include <arch/x86/x86intrin.h>
#include <assert.h>
#include <debug.h>
#include <kernel/spinlock.h>
#include <kernel/thread.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

void arch_thread_initialize(thread_t* t, vaddr_t entry_point) {
    // create a default stack frame on the stack
    vaddr_t stack_top = t->stack.top;

    // make sure the top of the stack is 16 byte aligned for ABI compliance
    stack_top = ROUNDDOWN(stack_top, 16);
    t->stack.top = stack_top;

    // make sure we start the frame 8 byte unaligned (relative to the 16 byte alignment) because
    // of the way the context switch will pop the return address off the stack. After the first
    // context switch, this leaves the stack unaligned relative to how a called function expects it.
    stack_top -= 8;
    struct x86_64_context_switch_frame* frame = (struct x86_64_context_switch_frame*)(stack_top);

    // Record a zero return address so that backtraces will stop here.
    // Otherwise if heap debugging is on, and say there is 99..99 here,
    // then the debugger could try to continue the backtrace from there.
    memset((void*)stack_top, 0, 8);

    // move down a frame size and zero it out
    frame--;
    memset(frame, 0, sizeof(*frame));

    frame->rip = entry_point;

    // initialize the saved extended register state
    vaddr_t buf = ROUNDUP(((vaddr_t)t->arch.extended_register_buffer), 64);
    __UNUSED size_t overhead = buf - (vaddr_t)t->arch.extended_register_buffer;
    DEBUG_ASSERT(sizeof(t->arch.extended_register_buffer) - overhead >=
                 x86_extended_register_size());
    t->arch.extended_register_state = (vaddr_t*)buf;
    x86_extended_register_init_state(t->arch.extended_register_state);

    // set the stack pointer
    t->arch.sp = (vaddr_t)frame;
#if __has_feature(safe_stack)
    t->arch.unsafe_sp =
        ROUNDDOWN(t->stack.unsafe_base + t->stack.size, 16);
#endif

    // initialize the fs, gs and kernel bases to 0.
    t->arch.fs_base = 0;
    t->arch.gs_base = 0;

    // Initialize the debug registers to a valid initial state.
    t->arch.track_debug_state = false;
    for (size_t i = 0; i < 4; i++) {
      t->arch.debug_state.dr[i] = 0;
    }
    t->arch.debug_state.dr6 = ~X86_DR6_USER_MASK;
    t->arch.debug_state.dr7 = ~X86_DR7_USER_MASK;
}

void arch_thread_construct_first(thread_t* t) {
}

void arch_dump_thread(thread_t* t) {
    if (t->state != THREAD_RUNNING) {
        dprintf(INFO, "\tarch: ");
        dprintf(INFO, "sp %#" PRIxPTR "\n", t->arch.sp);
    }
}

void* arch_thread_get_blocked_fp(struct thread* t) {
    if (!WITH_FRAME_POINTERS)
        return nullptr;

    struct x86_64_context_switch_frame* frame = (struct x86_64_context_switch_frame*)t->arch.sp;

    return (void*)frame->rbp;
}

__NO_SAFESTACK __attribute__((target("fsgsbase")))
void arch_context_switch(thread_t* oldthread, thread_t* newthread) {
    x86_extended_register_context_switch(oldthread, newthread);

    x86_debug_state_context_switch(oldthread, newthread);

    //printf("cs 0x%llx\n", kstack_top);

    /* set the tss SP0 value to point at the top of our stack */
    x86_set_tss_sp(newthread->stack.top);

    /* Save the user fs_base register value.  The new rdfsbase instruction
     * is much faster than reading the MSR, so use the former in
     * preference. */
    if (likely(g_x86_feature_fsgsbase)) {
        oldthread->arch.fs_base = _readfsbase_u64();
    } else {
        oldthread->arch.fs_base = read_msr(X86_MSR_IA32_FS_BASE);
    }

    /* The segment selector registers can't be preserved across context
     * switches in all cases, because some values get clobbered when
     * returning from interrupts.  If an interrupt occurs when a userland
     * process has set %fs = 1 (for example), the IRET instruction used for
     * returning from the interrupt will reset %fs to 0.
     *
     * To prevent the segment selector register values from leaking between
     * processes, we reset these registers across context switches. */
    set_ds(0);
    set_es(0);
    set_fs(0);
    if (get_gs() != 0) {
        /* Assigning to %gs clobbers gs_base, so we must restore gs_base
         * afterwards. */
        DEBUG_ASSERT(arch_ints_disabled());
        uintptr_t gs_base = (uintptr_t)x86_get_percpu();
        set_gs(0);
        write_msr(X86_MSR_IA32_GS_BASE, gs_base);
    }

    /* Restore fs_base and save+restore user gs_base.  Note that the user
     * and kernel gs_base values have been swapped -- the user value is
     * currently in KERNEL_GS_BASE. */
    if (likely(g_x86_feature_fsgsbase)) {
        /* There is no variant of the {rd,wr}gsbase instructions for
         * accessing KERNEL_GS_BASE, so we wrap those in two swapgs
         * instructions to get the same effect.  This is a little
         * convoluted, but still faster than using the KERNEL_GS_BASE
         * MSRs. */
        __asm__ __volatile__(
            "swapgs\n"
            "rdgsbase %[old_value]\n"
            "wrgsbase %[new_value]\n"
            "swapgs\n"
            : [old_value] "=&r"(oldthread->arch.gs_base)
            : [new_value] "r"(newthread->arch.gs_base));

        _writefsbase_u64(newthread->arch.fs_base);
    } else {
        oldthread->arch.gs_base = read_msr(X86_MSR_IA32_KERNEL_GS_BASE);
        write_msr(X86_MSR_IA32_FS_BASE, newthread->arch.fs_base);
        write_msr(X86_MSR_IA32_KERNEL_GS_BASE, newthread->arch.gs_base);
    }

#if __has_feature(safe_stack)
    oldthread->arch.unsafe_sp = x86_read_gs_offset64(ZX_TLS_UNSAFE_SP_OFFSET);
    x86_write_gs_offset64(ZX_TLS_UNSAFE_SP_OFFSET, newthread->arch.unsafe_sp);
#endif

    x86_64_context_switch(&oldthread->arch.sp, newthread->arch.sp);
}

void x86_debug_state_context_switch(thread_t *old_thread, thread_t *new_thread) {
    // If the new thread has debug state, then install it, replacing the current contents.
    if (unlikely(new_thread->arch.track_debug_state)) {
        // NOTE: There is no enable debug state call, as x86 doesn't have a global enable/disable
        //       switch, but rather enables particular registers through DR7. These registers are
        //       selected by userspace (and filtered by zircon) in the thread_write_state state
        //       syscall.
        //
        //       This means that just writing the thread debug state into the CPU is enough to
        //       activate the debug functionality.
        x86_write_hw_debug_regs(&new_thread->arch.debug_state);
        return;
    }

    // If the old thread had debug state running and the new one doesn't use it, disable the
    // debug capabilities.
    if (unlikely(old_thread->arch.track_debug_state)) {
        x86_disable_debug_state();
    }
}