xv6/Notes
2006-07-28 22:33:07 +00:00

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bochs 2.2.6:
./configure --enable-smp --enable-disasm --enable-debugger --enable-all-optimizations --enable-4meg-pages --enable-global-pages --enable-pae --disable-reset-on-triple-fault
bochs CVS after 2.2.6:
./configure --enable-smp --enable-disasm --enable-debugger --enable-all-optimizations --enable-4meg-pages --enable-global-pages --enable-pae
bootmain.c doesn't work right if the ELF sections aren't
sector-aligned. so you can't use ld -N. and the sections may also need
to be non-zero length, only really matters for tiny "kernels".
kernel loaded at 1 megabyte. stack same place that bootasm.S left it.
kinit() should find real mem size
and rescue useable memory below 1 meg
no paging, no use of page table hardware, just segments
no user area: no magic kernel stack mapping
so no copying of kernel stack during fork
though there is a kernel stack page for each process
no kernel malloc(), just kalloc() for user core
user pointers aren't valid in the kernel
setting up first process
we do want a process zero, as template
but not runnable
just set up return-from-trap frame on new kernel stack
fake user program that calls exec
map text read-only?
shared text?
what's on the stack during a trap or sys call?
PUSHA before scheduler switch? for callee-saved registers.
segment contents?
what does iret need to get out of the kernel?
how does INT know what kernel stack to use?
are interrupts turned on in the kernel? probably.
per-cpu curproc
one tss per process, or one per cpu?
one segment array per cpu, or per process?
pass curproc explicitly, or implicit from cpu #?
e.g. argument to newproc()?
hmm, you need a global curproc[cpu] for trap() &c
test stack expansion
test running out of memory, process slots
we can't really use a separate stack segment, since stack addresses
need to work correctly as ordinary pointers. the same may be true of
data vs text. how can we have a gap between data and stack, so that
both can grow, without committing 4GB of physical memory? does this
mean we need paging?
what's the simplest way to add the paging we need?
one page table, re-write it each time we leave the kernel?
page table per process?
probably need to use 0-0xffffffff segments, so that
both data and stack pointers always work
so is it now worth it to make a process's phys mem contiguous?
or could use segment limits and 4 meg pages?
but limits would prevent using stack pointers as data pointers
how to write-protect text? not important?
perhaps have fixed-size stack, put it in the data segment?
oops, if kernel stack is in contiguous user phys mem, then moving
users' memory (e.g. to expand it) will wreck any pointers into the
kernel stack.
do we need to set fs and gs? so user processes can't abuse them?
setupsegs() may modify current segment table, is that legal?
trap() ought to lgdt on return, since currently only done in swtch()
protect hardware interrupt vectors from user INT instructions?
test out-of-fd cases for creating pipe.
test pipe reader closes then write
test two readers, two writers.
test children being inherited by grandparent &c
some sleep()s should be interruptible by kill()
cli/sti in acquire/release should nest!
in case you acquire two locks
what would need fixing if we got rid of kernel_lock?
console output
proc_exit() needs lock on proc *array* to deallocate
kill() needs lock on proc *array*
allocator's free list
global fd table (really free-ness)
sys_close() on fd table
fork on proc list, also next pid
hold lock until public slots in proc struct initialized
locks
init_lock
sequences CPU startup
proc_table_lock
also protects next_pid
per-fd lock *just* protects count read-modify-write
also maybe freeness?
memory allocator
printf
wakeup needs proc_table_lock
so we need recursive locks?
or you must hold the lock to call wakeup?
in general, the table locks protect both free-ness and
public variables of table elements
in many cases you can use table elements w/o a lock
e.g. if you are the process, or you are using an fd
lock code shouldn't call cprintf...
nasty hack to allow locks before first process,
and to allow them in interrupts when curproc may be zero
race between release and sleep in sys_wait()
race between sys_exit waking up parent and setting state=ZOMBIE
race in pipe code when full/empty
lock order
per-pipe lock
proc_table_lock fd_table_lock kalloc_lock
console_lock
condition variable + mutex that protects it
proc * (for wait()), proc_table_lock
pipe structure, pipe lock
systematic way to test sleep races?
print something at the start of sleep?
do you have to be holding the mutex in order to call wakeup()?
device interrupts don't clear FL_IF
so a recursive timer interrupt is possible
what does inode->busy mean?
might be held across disk reads
no-one is allowed to do anything to the inode
protected by inode_table_lock
inode->count counts in-memory pointers to the struct
prevents inode[] element from being re-used
protected by inode_table_lock
blocks and inodes have ad-hoc sleep-locks
provide a single mechanism?
need to lock bufs in bio between bread and brelse
test 14-character file names
and file arguments longer than 14
and directories longer than one sector
kalloc() can return 0; do callers handle this right?