While working on the entry consolidation I stumbled over the KVM async page
fault handler and kvm_async_pf_task_wait() in particular. It took me a
while to realize that the randomly sprinkled around rcu_irq_enter()/exit()
invocations are just cargo cult programming. Several patches "fixed" RCU
splats by curing the symptoms without noticing that the code is flawed
from a design perspective.
The main problem is that this async injection is not based on a proper
handshake mechanism and only respects the minimal requirement, i.e. the
guest is not in a state where it has interrupts disabled.
Aside of that the actual code is a convoluted one fits it all swiss army
knife. It is invoked from different places with different RCU constraints:
1) Host side:
vcpu_enter_guest()
kvm_x86_ops->handle_exit()
kvm_handle_page_fault()
kvm_async_pf_task_wait()
The invocation happens from fully preemptible context.
2) Guest side:
The async page fault interrupted:
a) user space
b) preemptible kernel code which is not in a RCU read side
critical section
c) non-preemtible kernel code or a RCU read side critical section
or kernel code with CONFIG_PREEMPTION=n which allows not to
differentiate between #2b and #2c.
RCU is watching for:
#1 The vCPU exited and current is definitely not the idle task
#2a The #PF entry code on the guest went through enter_from_user_mode()
which reactivates RCU
#2b There is no preemptible, interrupts enabled code in the kernel
which can run with RCU looking away. (The idle task is always
non preemptible).
I.e. all schedulable states (#1, #2a, #2b) do not need any of this RCU
voodoo at all.
In #2c RCU is eventually not watching, but as that state cannot schedule
anyway there is no point to worry about it so it has to invoke
rcu_irq_enter() before running that code. This can be optimized, but this
will be done as an extra step in course of the entry code consolidation
work.
So the proper solution for this is to:
- Split kvm_async_pf_task_wait() into schedule and halt based waiting
interfaces which share the enqueueing code.
- Add comments (condensed form of this changelog) to spare others the
time waste and pain of reverse engineering all of this with the help of
uncomprehensible changelogs and code history.
- Invoke kvm_async_pf_task_wait_schedule() from kvm_handle_page_fault(),
user mode and schedulable kernel side async page faults (#1, #2a, #2b)
- Invoke kvm_async_pf_task_wait_halt() for the non schedulable kernel
case (#2c).
For this case also remove the rcu_irq_exit()/enter() pair around the
halt as it is just a pointless exercise:
- vCPUs can VMEXIT at any random point and can be scheduled out for
an arbitrary amount of time by the host and this is not any
different except that it voluntary triggers the exit via halt.
- The interrupted context could have RCU watching already. So the
rcu_irq_exit() before the halt is not gaining anything aside of
confusing the reader. Claiming that this might prevent RCU stalls
is just an illusion.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200505134059.262701431@linutronix.de
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