#include <stdint.h>
#include "stdio.h"
#include "mp/percpu.h"
#include "fs/vfs.h"
#include "syscall.h"

#define MSR_EFER          0xC0000080
#define MSR_STAR          0xC0000081
#define MSR_LSTAR         0xC0000082
#define MSR_SFMASK        0xC0000084
#define MSR_KERNEL_GSBASE 0xC0000102

#define EFER_SCE (1 << 0)

static struct cpu_local g_cpu_local;

static uint8_t g_syscall_kstack[16384] __attribute__((aligned(16)));

static inline void wrmsr(uint32_t msr, uint64_t value)
{
    asm volatile("wrmsr" :: "c"(msr), "a"((uint32_t)value), "d"((uint32_t)(value >> 32)));
}

static inline uint64_t rdmsr(uint32_t msr)
{
    uint32_t lo, hi;
    asm volatile("rdmsr" : "=a"(lo), "=d"(hi) : "c"(msr));
    return ((uint64_t)hi << 32) | lo;
}

uint64_t syscall_handler(uint64_t num,
                     uint64_t arg1, uint64_t arg2, uint64_t arg3,
                     uint64_t arg4, uint64_t arg5, uint64_t arg6)
{
    (void)arg4; (void)arg5; (void)arg6;
    switch (num)
    {   
        case SYS_READ:
        {
            int fd          = (int)arg1;
            uint8_t* buf    = (uint8_t*)arg2;
            size_t len      = (size_t)arg3;

            return (uint64_t)VFS_Read(fd, buf, len);
        }

        case SYS_WRITE:
        {
            int fd          = (int)arg1;
            uint8_t* buf    = (uint8_t*)arg2;
            size_t len      = (size_t)arg3;

            return (uint64_t)VFS_Write(fd, buf, len);
        }

        case SYS_OPEN:
        {
            const char* path = (const char*)arg1;
            return (uint64_t)VFS_Open(path);
        }
        
        case SYS_CLOSE:
        {
            int fd = (int)arg1;
            return (uint64_t)VFS_Close(fd);
        }

        default:
            return (uint64_t)-1;
    }
}

void syscall_init(void)
{
    extern void syscall_entry(void);

    // ── per-CPU local block ─────────────────────────────────────────────
    g_cpu_local.self       = (uint64_t)&g_cpu_local;
    g_cpu_local.kernel_rsp = (uint64_t)(g_syscall_kstack + sizeof(g_syscall_kstack));

    // After swapgs in syscall_entry, GS points here.
    wrmsr(MSR_KERNEL_GSBASE, (uint64_t)&g_cpu_local);

    // ── STAR ────────────────────────────────────────────────────────────
    // Bits [47:32]: SYSCALL loads CS = 0x08 (kernel code), SS = 0x10 (kernel data)
    // Bits [63:48]: SYSRET  loads CS = (base+16)|3 = 0x23, SS = (base+8)|3 = 0x1B
    //               → base must be 0x10
    uint64_t star = ((uint64_t)0x08 << 32) | ((uint64_t)0x10 << 48);
    wrmsr(MSR_STAR, star);

    wrmsr(MSR_LSTAR, (uint64_t)syscall_entry);

    uint64_t efer = rdmsr(MSR_EFER);
    efer |= EFER_SCE;
    wrmsr(MSR_EFER, efer);

    // Mask IF so interrupts are off during the syscall handler
    wrmsr(MSR_SFMASK, (1 << 9));
}