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ba0504600c
Currently the sched event handling may encounter data race problem, and
as a result some vcpus might be stalled forever.
One example can be wbinvd handling where more than 1 vcpus are doing
wbinvd concurrently. The following is a possible execution of 3 vcpus:
-------
0 1 2
req [Note: 0]
req bit0 set [Note: 1]
IPI -> 0
req bit2 set
IPI -> 2
VMExit
req bit2 cleared
wait
vcpu2 descheduled
VMExit
req bit0 cleared
wait
vcpu0 descheduled
signal 0
event0->set=true
wake 0
signal 2
event2->set=true [Note: 3]
wake 2
vcpu2 scheduled
event2->set=false
resume
req
req bit0 set
IPI -> 0
req bit1 set
IPI -> 1
(doesn't matter)
vcpu0 scheduled [Note: 4]
signal 0
event0->set=true
(no wake) [Note: 2]
event0->set=false (the rest doesn't matter)
resume
Any VMExit
req bit0 cleared
wait
idle running
(blocked forever)
Notes:
0: req: vcpu_make_request(vcpu, ACRN_REQUEST_WAIT_WBINVD).
1: req bit: Bit in pending_req_bits. Bit0 stands for bit for vcpu0.
2: In function signal_event, At this time the event->waiting_thread
is not NULL, so wake_thread will not execute
3: eventX: struct sched_event of vcpuX.
4: In function wait_event, the lock does not strictly cover the execution between
schedule() and event->set=false, so other threads may kick in.
-----
As shown in above example, before the last random VMExit, vcpu0 ended up
with request bit set but event->set==false, so blocked forever.
This patch proposes to change event->set from a boolean variable to an
integer. The semantic is very similar to a semaphore. The wait_event
will add 1 to this value, and block when this value is > 0, whereas signal_event
will decrease this value by 1.
It may happen that this value was decreased to a negative number but that
is OK. As long as the wait_event and signal_event are paired and
program order is observed (that is, wait_event always happens-before signal_event
on a single vcpu), this value will eventually be 0.
Tracked-On: #6405
Signed-off-by: Yifan Liu <yifan1.liu@intel.com>
ACRN Hypervisor ############### The open source `Project ACRN`_ defines a device hypervisor reference stack and an architecture for running multiple software subsystems, managed securely, on a consolidated system by means of a virtual machine manager. It also defines a reference framework implementation for virtual device emulation, called the "ACRN Device Model". The ACRN Hypervisor is a Type 1 reference hypervisor stack, running directly on the bare-metal hardware, and is suitable for a variety of IoT and embedded device solutions. The ACRN hypervisor addresses the gap that currently exists between datacenter hypervisors, and hard partitioning hypervisors. The ACRN hypervisor architecture partitions the system into different functional domains, with carefully selected guest OS sharing optimizations for IoT and embedded devices. You can find out more about Project ACRN on the `Project ACRN documentation`_ website. .. _`Project ACRN`: https://projectacrn.org .. _`ACRN Hypervisor`: https://github.com/projectacrn/acrn-hypervisor .. _`Project ACRN documentation`: https://projectacrn.github.io/