not doing a reformat of the entire project to match
nightly formatting changes, but capturing the published
crates so we can maybe save a version bump later on to get
the project clean.
We no longer send messages to the swapper via the server interface.
All calls go through the blocking handler; in general, there are
no scenarios where it makes sense to have a swapper operation
pre-emptable. We always want our swapper ops to complete quickly,
deterministically, and atomically.
I think it's more Xous-y to punt to userland in the case that
no consecutive memory is found, rather than trying to implement
an evictor in the kernel itself.
For now, the blunt hammer we use is to simply request some
blocks be freed and we retry; this works well enough to find
blocks of a few pages in sequence. If we needed larger blocks,
we may want to consider modifying the swapper userland algorithm
to have some API for finding consecutive runs of non-wired
memory to swap out.
"Device RAM" is a block of RAM whose physical address is not
important, but the block must be consecutively allocated in
physical address space.
This is used to allocate blocks of memory that are intended
for hardware DMA targets and other peripheral shared-memory
applications.
Note that this also marks the allocated block as "wired", so
it can't be swapped out.
apply the same techniques in the loader key generator to
the kernel TRNG. Every pull of data from the kernel reseeds
the TRNG state as well. This makes server ID creation somewhat
more expensive, but should greatly improve the security of
the system in the case of a TRNG with occassional drop-outs.
Shoot for 256 bits minimum entropy in the seed pool, gives us
some safety margin over our required 128.
note that the pool only gets more entropy over time, as every
call to the pool folds more entropy in (roughly 2 bits per call).
In this situation, we enter the OOM with the hardware address
space vs the process space inconsistent (the hardware address
space was transitioned in anticipation of mapping a block into it).
This patch reverts our address space to the original space then
runs the OOM routine, and then puts us back where we were.
previously we could re-enter into a process through a divergent
call into the swapper. Now, we only ever re-enter as a syscall
through the syscall interface.
in order to make this happen we had to store all the thread and
stack state for the handler so we can do a re-entrant syscall, but
I think this is worth it. makes the code so much better.
- makes retrieve_page in-lineable into existing kernel routines
without a divergence issue on the call return
- reduces code duplication
- fixes a hairy bug on interrupt restore timing. With the previous
method, the interrupts would have to un-gate just before the
syscall executes its return to normalcy; this can cause a race
condition between a hardware interrupt and that return context
switch which can lead to the return context being corrupted.
By making this re-entrant, we can make the IRQ pause/restore
point happen outside of any fancy system context footwork, which
makes the interrupt via hardware source work again.
in practice the hard-OOM only seems to happen, and oom-doom
takes up precious memory that could be used for preventing
OOMs. Just leave one path in, but make it a bit smarter.
The smarts are still in userland, so the kernel is minimal
dependencies.
Re-entrant syscall is tricky because we end up smashing
the return thread context and stack of the current syscall.
We stash the thread context and stack in order to make this
happen, but we have to make some assumption about the potential
stack size. There's a assert to save us if this is wrong, but
generally the kernel is extremely well behaved on stack consumption
so we shouldn't be bumping into it.
This version of the OOM handler allows us to bury the OOM
handler "anywhere", which means we can handle the hard OOM
in multiple allocation methods without having to track the entry
and exit point of every single possible one.
This means we can chop a lot of fragile code out of the kernel.
We'll do that after this commit, and some testing, to make sure
this is the right path, but it feels "much nicer".
We'll probably also cut the oom-doomer, because I think in
practice it won't be effective and it consumes a non-trivial amount
of RAM, which would otherwise make OOMs more infrequent -- probably
an amount that would be greater than the benefit that any soft-OOM
handler can get us.
The Hard OOM re-entrant syscall can actually originate in
any process space, but only from inside the kernel. Modify the
checks so that the PID==2 check is applied only to all the other
calls; and that a supervisor mode check is applied to the hard OOM
call.
This version of the OOM handler uses a syscall in a syscall
(previously forbidden) to make the OOM case deep in an alloc
situation oomable, but also have multiple exit paths.
this saves us from having to recode oodles of exit paths which
is complicated and error-prone.
I also think we should trim back some of the OOM handling paths
because it is bloating the swapper, but let's commit the current
working state before we trim stuff.
turns out that
- hard OOM has a clearly defined threshold: 0. we shouldn't
make it higher, because then it forces us to count free memory
on every allocation.
- it's better to have the tuning in userspace (I think) for
OOM, because that's where all the algorithms are
