p13-dev-fs Device Virtual Filesystem Integration

[Yash-Rawat-IIT-D]

p13-dev-fs Device Virtual Filesystem

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Branch: p13-dev-fs
Type: Original implementation
Impact: Enables device-as-file abstraction with console I/O through file operations

Context:
Implemented device virtual filesystem allowing devices to be accessed through standard file operations (open/read/write). Built on p12-log (PR #22 ). Follows C implementation semantics from p13-dev-fs.md specification.

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Changes Made:

1. Device Constants (src/param.rs)

   • Added NDEV = 10 (maximum device numbers)
   • Added CONSOLE = 1 (console device major number)

2. Device Switch Table (src/file.rs)

   • Created Devsw struct with function pointer fields:

     pub struct Devsw {
         pub read: Option<fn(usize, &mut [u8], i32) -> i32>,
         pub write: Option<fn(usize, &[u8], i32) -> i32>,
     }

   • Declared static mut DEVSW: [Devsw; NDEV] global device table
   • Added FileType::Device variant to enum (was only None, Inode)
   • Implemented mknod(path, major, minor) function:
     • Wraps filesystem operations in begin_op()/end_op() transactions
     • Calls create() with T_DEV type and device numbers
     • Returns 0 on success, -1 on failure (C semantic equivalence)

3. Console Device Handlers (src/console.rs)

   • Implemented consoleread(ip, dst, n) -> i32:
     • Blocking wait for input with volatile reads (critical fix)
     • Reads from INPUT buffer circular queue (r/w/e indices)
     • Handles Ctrl-D EOF with preservation for next call
     • Breaks on newline character
     • Returns bytes read
   • Implemented consolewrite(ip, src, n) -> i32:
     • Iterates through source buffer calling consputc()
     • Returns byte count
   • Implemented consoleinit():
     • Registers consoleread/consolewrite in DEVSW[CONSOLE]
     • Called during boot initialization

4. Device I/O Routing (src/fs.rs)

   • Modified readi() to check for T_DEV type (early return before offset validation):

     if ip.type_ == T_DEV as i16 {
         if ip.major < 0 || (ip.major as usize) >= NDEV || DEVSW[...].read.is_none() {
             return -1;
         }
         return DEVSW[ip.major as usize].read.unwrap()(idx, dst, n as i32);
     }

   • Modified writei() with identical pattern for device write dispatch
   • Added imports: T_DEV, NDEV, DEVSW
   • Bounds checking and handler validation before dispatch

5. Boot Initialization (src/lib.rs)

   • Added console::consoleinit() after ioapic_init() (registers handlers)
   • Added file::mknod("/console", CONSOLE, CONSOLE) after initlog() (creates device file)
   • Rewrote welcome() function to test console device:

     let c = file::open("/console", O_RDWR).unwrap_or_else(...);
     file::filewrite(c, b"\nEnter your name: ", 18);
     let mut name = [0u8; 20];
     let namelen = file::fileread(c, &mut name, 20);
     file::filewrite(c, b"Nice to meet you! ", 18);
     file::filewrite(c, &name[..namelen as usize], namelen);
     file::filewrite(c, b"BYE!\n", 5);
     file::fileclose(c);

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Critical Bug/Implementation Fixes

1. Compiler Optimization Blocking

   • Issue: consoleread() hung indefinitely in spin loop despite interrupt handler updating INPUT.w
   • Root cause: Compiler cached INPUT.r and INPUT.w values (tight loop with no modifications visible to optimizer)
   • Debug evidence: Interrupt handler logs showed w updating from 0 to 6, but read loop never observed change
   • Fix: Added core::ptr::read_volatile() to force re-reading on every iteration:

     while core::ptr::read_volatile(&(*input).r) == core::ptr::read_volatile(&(*input).w) {
         core::hint::spin_loop();
     }

   • Result: Loop correctly detects interrupt handler updates

2. Byte Count Mismatch

   • Issue: "BYE!\n" not appearing in output
   • Root cause: Passed n=6 for 5-byte string (reading garbage byte)
   • Fix: Changed to n=5 or use .len() as i32

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Constraints Met:

• Semantic equivalence with C implementation (control flow preserved)
• Device routing in fs.rs matches C placement (before offset checks)
• Transaction wrapping in mknod() matches C pattern
• All string literals and byte counts verified against C version

Testing:

• Platform: QEMU on Linux (WSL)
• Build: 0 compilation warnings, 0 errors
• Runtime validation:
  • Console device created at boot (/console in root directory)
  • welcome() prompts for name input
  • User types name and presses Enter
  • System echoes: "Nice to meet you! [name]"
  • Displays "BYE!" and exits
• Input properly buffered until newline