//! PVH Boot Protocol Implementation
//!
//! PVH (Para-Virtualized Hardware) is a boot protocol that allows direct kernel
//! entry without BIOS/UEFI firmware. This is the fastest path to boot a Linux VM.
//!
//! # Overview
//!
//! The PVH boot protocol:
//! 1. Skips BIOS POST and firmware initialization
//! 2. Loads kernel directly into memory
//! 3. Sets up minimal boot structures (E820 map, start_info)
//! 4. Jumps directly to kernel 64-bit entry point
//!
//! # Boot Time Comparison
//!
//! | Method | Boot Time |
//! |--------|-----------|
//! | BIOS   | 1-3s      |
//! | UEFI   | 0.5-1s    |
//! | PVH    | <50ms     |
//!
//! # Memory Requirements
//!
//! The PVH start_info structure must be placed in guest memory and
//! its address passed to the kernel via RBX register.

use super::{layout, BootError, GuestMemory, Result};

/// Maximum number of E820 entries
pub const MAX_E820_ENTRIES: usize = 128;

/// E820 memory type values (matching Linux kernel definitions)
#[repr(u32)]
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum E820Type {
    /// Usable RAM
    Ram = 1,
    /// Reserved by system
    Reserved = 2,
    /// ACPI reclaimable
    Acpi = 3,
    /// ACPI NVS (Non-Volatile Storage)
    Nvs = 4,
    /// Unusable memory
    Unusable = 5,
    /// Disabled memory (EFI)
    Disabled = 6,
    /// Persistent memory
    Pmem = 7,
    /// Undefined/other
    Undefined = 0,
}

impl From<u32> for E820Type {
    fn from(val: u32) -> Self {
        match val {
            1 => E820Type::Ram,
            2 => E820Type::Reserved,
            3 => E820Type::Acpi,
            4 => E820Type::Nvs,
            5 => E820Type::Unusable,
            6 => E820Type::Disabled,
            7 => E820Type::Pmem,
            _ => E820Type::Undefined,
        }
    }
}

/// E820 memory map entry
///
/// Matches the Linux kernel's e820entry structure for compatibility.
#[repr(C, packed)]
#[derive(Debug, Clone, Copy, Default)]
pub struct E820Entry {
    /// Start address of memory region
    pub addr: u64,
    /// Size of memory region in bytes
    pub size: u64,
    /// Type of memory region
    pub entry_type: u32,
}

impl E820Entry {
    /// Create a new E820 entry
    pub fn new(addr: u64, size: u64, entry_type: E820Type) -> Self {
        Self {
            addr,
            size,
            entry_type: entry_type as u32,
        }
    }

    /// Create a RAM entry
    pub fn ram(addr: u64, size: u64) -> Self {
        Self::new(addr, size, E820Type::Ram)
    }

    /// Create a reserved entry
    pub fn reserved(addr: u64, size: u64) -> Self {
        Self::new(addr, size, E820Type::Reserved)
    }
}

/// PVH start_info structure
///
/// This is a simplified version compatible with the Xen PVH ABI.
/// The structure is placed in guest memory and its address is passed
/// to the kernel in RBX.
///
/// # Memory Layout
///
/// The structure must be at a known location (typically 0x7000) and
/// contain pointers to other boot structures.
#[repr(C)]
#[derive(Debug, Clone, Default)]
pub struct StartInfo {
    /// Magic number (XEN_HVM_START_MAGIC_VALUE or custom)
    pub magic: u32,
    /// Version of the start_info structure
    pub version: u32,
    /// Flags (reserved, should be 0)
    pub flags: u32,
    /// Number of modules (initrd counts as 1)
    pub nr_modules: u32,
    /// Physical address of module list
    pub modlist_paddr: u64,
    /// Physical address of command line string
    pub cmdline_paddr: u64,
    /// Physical address of RSDP (ACPI, 0 if none)
    pub rsdp_paddr: u64,
    /// Physical address of E820 memory map
    pub memmap_paddr: u64,
    /// Number of entries in memory map
    pub memmap_entries: u32,
    /// Reserved/padding
    pub reserved: u32,
}

/// XEN HVM start magic value
pub const XEN_HVM_START_MAGIC: u32 = 0x336ec578;

/// Volt custom magic (for identification)
pub const VOLT_MAGIC: u32 = 0x4e4f5641; // "NOVA"

impl StartInfo {
    /// Create a new StartInfo with default values
    pub fn new() -> Self {
        Self {
            magic: XEN_HVM_START_MAGIC,
            version: 1,
            flags: 0,
            ..Default::default()
        }
    }

    /// Set command line address
    pub fn with_cmdline(mut self, addr: u64) -> Self {
        self.cmdline_paddr = addr;
        self
    }

    /// Set memory map address and entry count
    pub fn with_memmap(mut self, addr: u64, entries: u32) -> Self {
        self.memmap_paddr = addr;
        self.memmap_entries = entries;
        self
    }

    /// Set module (initrd) information
    pub fn with_module(mut self, modlist_addr: u64) -> Self {
        self.nr_modules = 1;
        self.modlist_paddr = modlist_addr;
        self
    }

    /// Convert to bytes for writing to guest memory
    pub fn as_bytes(&self) -> &[u8] {
        unsafe {
            std::slice::from_raw_parts(
                self as *const Self as *const u8,
                std::mem::size_of::<Self>(),
            )
        }
    }
}

/// Module (initrd) entry for PVH
#[repr(C)]
#[derive(Debug, Clone, Copy, Default)]
pub struct HvmModlistEntry {
    /// Physical address of module
    pub paddr: u64,
    /// Size of module in bytes
    pub size: u64,
    /// Physical address of command line for module (0 if none)
    pub cmdline_paddr: u64,
    /// Reserved
    pub reserved: u64,
}

impl HvmModlistEntry {
    /// Create entry for initrd
    pub fn new(paddr: u64, size: u64) -> Self {
        Self {
            paddr,
            size,
            cmdline_paddr: 0,
            reserved: 0,
        }
    }

    /// Convert to bytes
    pub fn as_bytes(&self) -> &[u8] {
        unsafe {
            std::slice::from_raw_parts(
                self as *const Self as *const u8,
                std::mem::size_of::<Self>(),
            )
        }
    }
}

/// PVH configuration for boot setup
#[derive(Debug, Clone)]
pub struct PvhConfig {
    /// Total memory size in bytes
    pub memory_size: u64,
    /// Number of vCPUs
    pub vcpu_count: u32,
    /// Physical address of command line
    pub cmdline_addr: u64,
    /// Physical address of initrd (if any)
    pub initrd_addr: Option<u64>,
    /// Size of initrd (if any)
    pub initrd_size: Option<u64>,
}

/// PVH boot setup implementation
pub struct PvhBootSetup;

impl PvhBootSetup {
    /// Set up PVH boot structures in guest memory
    ///
    /// Creates and writes:
    /// 1. E820 memory map
    /// 2. start_info structure
    /// 3. Module list (for initrd)
    pub fn setup<M: GuestMemory>(config: &PvhConfig, guest_mem: &mut M) -> Result<()> {
        // Build E820 memory map
        let e820_entries = Self::build_e820_map(config.memory_size)?;
        let e820_count = e820_entries.len() as u32;

        // Write E820 map to guest memory
        Self::write_e820_map(&e820_entries, guest_mem)?;

        // Write module list if initrd is present
        let modlist_addr = if let (Some(addr), Some(size)) = (config.initrd_addr, config.initrd_size) {
            let modlist_addr = layout::E820_MAP_ADDR + 
                (MAX_E820_ENTRIES * std::mem::size_of::<E820Entry>()) as u64;
            
            let entry = HvmModlistEntry::new(addr, size);
            guest_mem.write_bytes(modlist_addr, entry.as_bytes())?;
            
            Some(modlist_addr)
        } else {
            None
        };

        // Build and write start_info structure
        let mut start_info = StartInfo::new()
            .with_cmdline(config.cmdline_addr)
            .with_memmap(layout::E820_MAP_ADDR, e820_count);

        if let Some(addr) = modlist_addr {
            start_info = start_info.with_module(addr);
        }

        guest_mem.write_bytes(layout::PVH_START_INFO_ADDR, start_info.as_bytes())?;

        Ok(())
    }

    /// Build E820 memory map for the VM
    ///
    /// Creates a standard x86_64 memory layout:
    /// - Low memory (0-640KB): RAM
    /// - Legacy hole (640KB-1MB): Reserved
    /// - High memory (1MB+): RAM
    fn build_e820_map(memory_size: u64) -> Result<Vec<E820Entry>> {
        let mut entries = Vec::with_capacity(4);

        // Validate minimum memory
        if memory_size < layout::HIGH_MEMORY_START {
            return Err(BootError::MemoryLayout(format!(
                "Memory size {} is less than minimum required {}",
                memory_size,
                layout::HIGH_MEMORY_START
            )));
        }

        // Low memory: 0 to 640KB (0x0 - 0x9FFFF)
        // We reserve the first page for real-mode IVT
        entries.push(E820Entry::ram(0, layout::LOW_MEMORY_END));

        // Legacy video/ROM hole: 640KB to 1MB (0xA0000 - 0xFFFFF)
        // This is reserved for VGA memory, option ROMs, etc.
        let legacy_hole_size = layout::HIGH_MEMORY_START - layout::LOW_MEMORY_END;
        entries.push(E820Entry::reserved(layout::LOW_MEMORY_END, legacy_hole_size));

        // High memory: 1MB to RAM size
        let high_memory_size = memory_size - layout::HIGH_MEMORY_START;
        if high_memory_size > 0 {
            entries.push(E820Entry::ram(layout::HIGH_MEMORY_START, high_memory_size));
        }

        // If memory > 4GB, we might need to handle the MMIO hole
        // For now, we assume memory <= 4GB for simplicity
        // Production systems should handle:
        // - PCI MMIO hole (typically 0xE0000000 - 0xFFFFFFFF)
        // - Memory above 4GB remapped

        Ok(entries)
    }

    /// Write E820 map entries to guest memory
    fn write_e820_map<M: GuestMemory>(entries: &[E820Entry], guest_mem: &mut M) -> Result<()> {
        let entry_size = std::mem::size_of::<E820Entry>();
        
        for (i, entry) in entries.iter().enumerate() {
            let addr = layout::E820_MAP_ADDR + (i * entry_size) as u64;
            let bytes = unsafe {
                std::slice::from_raw_parts(entry as *const E820Entry as *const u8, entry_size)
            };
            guest_mem.write_bytes(addr, bytes)?;
        }

        Ok(())
    }

    /// Get initial CPU register state for PVH boot
    ///
    /// Returns the register values needed to start the vCPU in 64-bit mode
    /// with PVH boot protocol.
    pub fn get_initial_regs(entry_point: u64) -> PvhRegs {
        PvhRegs {
            // Instruction pointer - kernel entry
            rip: entry_point,
            
            // RBX contains pointer to start_info (Xen PVH convention)
            rbx: layout::PVH_START_INFO_ADDR,
            
            // RSI also contains start_info pointer (Linux boot convention)
            rsi: layout::PVH_START_INFO_ADDR,
            
            // Stack pointer
            rsp: layout::BOOT_STACK_POINTER,
            
            // Clear other general-purpose registers
            rax: 0,
            rcx: 0,
            rdx: 0,
            rdi: 0,
            rbp: 0,
            r8: 0,
            r9: 0,
            r10: 0,
            r11: 0,
            r12: 0,
            r13: 0,
            r14: 0,
            r15: 0,
            
            // Flags - interrupts disabled
            rflags: 0x2,
            
            // Segment selectors for 64-bit mode
            cs: 0x10,  // Code segment, ring 0
            ds: 0x18,  // Data segment
            es: 0x18,
            fs: 0x18,
            gs: 0x18,
            ss: 0x18,
            
            // CR registers for 64-bit mode
            cr0: CR0_PE | CR0_ET | CR0_PG,
            cr3: 0,  // Page table base - set by kernel setup
            cr4: CR4_PAE,
            
            // EFER for long mode
            efer: EFER_LME | EFER_LMA,
        }
    }
}

/// Control Register 0 bits
const CR0_PE: u64 = 1 << 0;   // Protection Enable
const CR0_ET: u64 = 1 << 4;   // Extension Type (387 present)
const CR0_PG: u64 = 1 << 31;  // Paging Enable

/// Control Register 4 bits
const CR4_PAE: u64 = 1 << 5;  // Physical Address Extension

/// EFER (Extended Feature Enable Register) bits
const EFER_LME: u64 = 1 << 8;  // Long Mode Enable
const EFER_LMA: u64 = 1 << 10; // Long Mode Active

/// CPU register state for PVH boot
#[derive(Debug, Clone, Default)]
pub struct PvhRegs {
    // General purpose registers
    pub rax: u64,
    pub rbx: u64,
    pub rcx: u64,
    pub rdx: u64,
    pub rsi: u64,
    pub rdi: u64,
    pub rsp: u64,
    pub rbp: u64,
    pub r8: u64,
    pub r9: u64,
    pub r10: u64,
    pub r11: u64,
    pub r12: u64,
    pub r13: u64,
    pub r14: u64,
    pub r15: u64,
    
    // Instruction pointer
    pub rip: u64,
    
    // Flags
    pub rflags: u64,
    
    // Segment selectors
    pub cs: u16,
    pub ds: u16,
    pub es: u16,
    pub fs: u16,
    pub gs: u16,
    pub ss: u16,
    
    // Control registers
    pub cr0: u64,
    pub cr3: u64,
    pub cr4: u64,
    
    // Model-specific registers
    pub efer: u64,
}

/// GDT entries for 64-bit mode boot
///
/// This provides a minimal GDT for transitioning to 64-bit mode.
/// The kernel will set up its own GDT later.
pub struct BootGdt;

impl BootGdt {
    /// Null descriptor (required as GDT[0])
    pub const NULL: u64 = 0;
    
    /// 64-bit code segment (CS)
    /// Base: 0, Limit: 0xFFFFF (ignored in 64-bit mode)
    /// Type: Code, Execute/Read, Present, DPL=0
    pub const CODE64: u64 = 0x00af_9b00_0000_ffff;
    
    /// 64-bit data segment (DS, ES, SS, FS, GS)
    /// Base: 0, Limit: 0xFFFFF
    /// Type: Data, Read/Write, Present, DPL=0
    pub const DATA64: u64 = 0x00cf_9300_0000_ffff;
    
    /// Build GDT table as bytes
    pub fn as_bytes() -> [u8; 24] {
        let mut gdt = [0u8; 24];
        gdt[0..8].copy_from_slice(&Self::NULL.to_le_bytes());
        gdt[8..16].copy_from_slice(&Self::CODE64.to_le_bytes());
        gdt[16..24].copy_from_slice(&Self::DATA64.to_le_bytes());
        gdt
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    struct MockMemory {
        size: u64,
        data: Vec<u8>,
    }

    impl MockMemory {
        fn new(size: u64) -> Self {
            Self {
                size,
                data: vec![0; size as usize],
            }
        }
    }

    impl GuestMemory for MockMemory {
        fn write_bytes(&mut self, addr: u64, data: &[u8]) -> Result<()> {
            let end = addr as usize + data.len();
            if end > self.data.len() {
                return Err(BootError::GuestMemoryWrite(format!(
                    "Write at {:#x} exceeds memory size",
                    addr
                )));
            }
            self.data[addr as usize..end].copy_from_slice(data);
            Ok(())
        }

        fn size(&self) -> u64 {
            self.size
        }
    }

    #[test]
    fn test_e820_entry_size() {
        // E820 entry must be exactly 20 bytes for Linux kernel compatibility
        assert_eq!(std::mem::size_of::<E820Entry>(), 20);
    }

    #[test]
    fn test_build_e820_map() {
        let memory_size = 128 * 1024 * 1024; // 128MB
        let entries = PvhBootSetup::build_e820_map(memory_size).unwrap();

        // Should have at least 3 entries
        assert!(entries.len() >= 3);

        // First entry should be low memory RAM — copy from packed struct
        let e0_addr = entries[0].addr;
        let e0_type = entries[0].entry_type;
        assert_eq!(e0_addr, 0);
        assert_eq!(e0_type, E820Type::Ram as u32);

        // Second entry should be legacy hole (reserved)
        let e1_addr = entries[1].addr;
        let e1_type = entries[1].entry_type;
        assert_eq!(e1_addr, layout::LOW_MEMORY_END);
        assert_eq!(e1_type, E820Type::Reserved as u32);

        // Third entry should be high memory RAM
        let e2_addr = entries[2].addr;
        let e2_type = entries[2].entry_type;
        assert_eq!(e2_addr, layout::HIGH_MEMORY_START);
        assert_eq!(e2_type, E820Type::Ram as u32);
    }

    #[test]
    fn test_start_info_size() {
        // StartInfo should be reasonable size (under 4KB page)
        let size = std::mem::size_of::<StartInfo>();
        assert!(size < 4096);
        assert!(size >= 48); // Minimum expected fields
    }

    #[test]
    fn test_pvh_setup() {
        let mut mem = MockMemory::new(128 * 1024 * 1024);
        let config = PvhConfig {
            memory_size: 128 * 1024 * 1024,
            vcpu_count: 2,
            cmdline_addr: layout::CMDLINE_ADDR,
            initrd_addr: Some(100 * 1024 * 1024),
            initrd_size: Some(10 * 1024 * 1024),
        };

        let result = PvhBootSetup::setup(&config, &mut mem);
        assert!(result.is_ok());

        // Verify magic was written to start_info location
        let magic = u32::from_le_bytes([
            mem.data[layout::PVH_START_INFO_ADDR as usize],
            mem.data[layout::PVH_START_INFO_ADDR as usize + 1],
            mem.data[layout::PVH_START_INFO_ADDR as usize + 2],
            mem.data[layout::PVH_START_INFO_ADDR as usize + 3],
        ]);
        assert_eq!(magic, XEN_HVM_START_MAGIC);
    }

    #[test]
    fn test_pvh_regs() {
        let entry_point = 0x100200;
        let regs = PvhBootSetup::get_initial_regs(entry_point);

        // Verify entry point
        assert_eq!(regs.rip, entry_point);

        // Verify start_info pointer in rbx
        assert_eq!(regs.rbx, layout::PVH_START_INFO_ADDR);

        // Verify 64-bit mode flags
        assert!(regs.cr0 & CR0_PE != 0); // Protection enabled
        assert!(regs.cr0 & CR0_PG != 0); // Paging enabled
        assert!(regs.cr4 & CR4_PAE != 0); // PAE enabled
        assert!(regs.efer & EFER_LME != 0); // Long mode enabled
    }

    #[test]
    fn test_gdt_layout() {
        let gdt = BootGdt::as_bytes();
        assert_eq!(gdt.len(), 24); // 3 entries × 8 bytes

        // First entry should be null
        assert_eq!(&gdt[0..8], &[0u8; 8]);
    }
}