to dequeue a packet.
The tx path was trying to ensure that enough Xenbus TX ring slots existed but
it didn't check to see whether the mbuf TX ring slots were also available.
They get freed in xn_txeof() which occurs after transmission, rather than earlier
on in the process. (The same happens under Linux too.)
Due to whatever reason (CPU use, scheduling, memory constraints, whatever) the
mbuf TX ring may not have enough slots free and would allocate slot 0. This is
used as the freelist head pointer to represent "free" mbuf TX ring slots; setting
this to an actual mbuf value rather than an id crashes the code.
This commit introduces some basic code to track the TX mbuf ring use and then
(hopefully!) ensures that enough slots are free in said TX mbuf ring before it
enters the actual work loop.
A few notes:
* Similar logic needs to be introduced to check there are enough actual slots
available in the xenbuf TX ring. There's some logic which is invoked earlier
but it doesn't hard-check against the number of available ring slots.
Its trivial to do; I'll do it in a subsequent commit.
* As I've now commented in the source, it is likely possible to deadlock the
driver under certain conditions where the rings aren't receiving any changes
(which I should enumerate) and thus Xen doesn't send any further software
interrupts. I need to make sure that the timer(s) are running right and
the queues are periodically kicked.
PR: 134926
Slot 0 must always remain "free" and be a pointer to the first free entry in the
mbuf descriptor list. It is thus an error to have code allocate or push slot 0
back into the list.
Log:
- merge in latest xenbus from dfr's xenhvm
- fix race condition in xs_read_reply by converting tsleep to mtx_sleep
Log:
unmask evtchn in bind_{virq, ipi}_to_irq
Log:
- remove code for handling case of not being able to sleep
- eliminate tsleep - make sleeps atomic
