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//! System Call Entry Point
//! Assembly stub for handling SYSCALL instruction from userspace
// External handlers
extern fn handle_syscall(ctx: u64) void;
extern fn get_kernel_stack() u64;
/// Address of syscall entry point for LSTAR MSR
pub fn get_entry_address() u64 {
return @intFromPtr(&syscall_entry_asm);
}
extern fn syscall_entry_asm() void;
comptime {
asm (
\\.global syscall_entry_asm
\\syscall_entry_asm:
\\ # SYSCALL has already:
\\ # - Saved RIP to RCX
\\ # - Saved RFLAGS to R11
\\ # - Loaded CS/SS from STAR
\\ # - Jumped here (LSTAR)
\\ # - Masked RFLAGS per FMASK
\\ #
\\ # We need to:
\\ # - Save user RSP
\\ # - Switch to kernel stack (from TSS)
\\ # - Save all registers
\\ # - Call handler
\\ # - Restore registers
\\ # - Use sysret to return
\\
\\ # At entry: RCX = user RIP, R11 = user RFLAGS, RSP = user RSP, RAX = syscall#
\\
\\ # Strategy: Use SWAPGS-like approach but with stack
\\ # Save everything on user stack, switch to kernel stack, copy over
\\
\\ # First, save user RSP by pushing all regs then calculating
\\ push %rax
\\ push %rbx
\\ push %rcx
\\ push %rdx
\\ push %rsi
\\ push %rdi
\\ push %rbp
\\ push %r8
\\ push %r9
\\ push %r10
\\ push %r11
\\ push %r12
\\ push %r13
\\ push %r14
\\ push %r15
\\
\\ # Get kernel stack (returns in RAX, RSP unchanged after ret)
\\ call get_kernel_stack
\\
\\ # RAX now has kernel stack pointer
\\ # RSP points to saved R15 (call/ret balanced)
\\ mov %rsp, %r15 # R15 = pointer to saved registers
\\ lea 120(%r15), %r14 # R14 = original user RSP (before 15 pushes)
\\ mov %rax, %rsp # Switch to kernel stack
\\
\\ # Build struct on kernel stack by reading from user stack (via R15)
\\ # User stack has (from low to high addr): [rax][rbx][rcx][rdx][rsi][rdi][rbp][r8-r15]
\\ # We push in REVERSE order so struct layout matches (stack grows down)
\\ # NOTE: RCX and R11 already have user_rip and user_rflags from SYSCALL!
\\ push %r14 # user_rsp (last in struct, push first)
\\ push 32(%r15) # user_rflags (R11 value before syscall)
\\ push $0x1B # user_ss
\\ push 96(%r15) # user_rip (RCX value before syscall)
\\ push $0x23 # user_cs
\\ push 0(%r15) # r15
\\ push 8(%r15) # r14
\\ push 16(%r15) # r13
\\ push 24(%r15) # r12
\\ push 32(%r15) # r11 (value before syscall)
\\ push 40(%r15) # r10
\\ push 48(%r15) # r9
\\ push 56(%r15) # r8
\\ push 64(%r15) # rbp
\\ push 72(%r15) # rdi
\\ push 80(%r15) # rsi
\\ push 88(%r15) # rdx
\\ push 96(%r15) # rcx (value before syscall)
\\ push 104(%r15) # rbx
\\ push 112(%r15) # rax (first in struct, push last)
\\
\\ # Re-enable interrupts (we're on kernel stack now)
\\ sti
\\
\\ # Call invocation handler
\\ mov %rsp, %rdi # Pass context pointer
\\ call handle_syscall
\\
\\ # Disable interrupts before returning to user
\\ cli
\\
\\ # Restore registers (RAX might be modified with return value)
\\ # NOTE: Don't restore RCX and R11 - they're needed for sysret!
\\ pop %rax
\\ pop %rbx
\\ add $8, %rsp # Skip RCX (will be loaded from saved RIP)
\\ pop %rdx
\\ pop %rsi
\\ pop %rdi
\\ pop %rbp
\\ pop %r8
\\ pop %r9
\\ pop %r10
\\ add $8, %rsp # Skip R11 (will be loaded from saved RFLAGS)
\\ pop %r12
\\ pop %r13
\\ pop %r14
\\ pop %r15
\\
\\ # Pop saved context
\\ add $8, %rsp # Skip user CS (sysret will set it)
\\ pop %rcx # User RIP for sysret
\\ add $8, %rsp # Skip user SS (sysret will set it)
\\ pop %r11 # User RFLAGS for sysret
\\ pop %rsp # Restore user RSP
\\
\\ # Return to userspace
\\ sysretq
);
}
|
