On 4/25/25 00:08, Sebastian Andrzej Siewior wrote:
On 2025-04-24 21:27:59 [+0530], Shrikanth Hegde wrote:
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 19f4d298d..123539642 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -4901,7 +4901,7 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64
time_limit,
}
if (need_resched())
- cond_resched();
+ schedule();
This looks unrelated and odd. I don't why but this should be a
cond_resched() so it can be optimized away on PREEMPT kernels.
This is needed, otherwise KVM on powerVM setup gets stuck on preempt=full/lazy.
But this makes no sense. On preempt=full the cond_resched() gets patched
out while schedule() doesn't. Okay, this explains the stuck.
cond_resched works. What you said is right about schedule and preemption models.
Initially I had some other code changes and they were causing it get stuck. i
retested it.
But looking at the semantics of usage of xfer_to_guest_mode_work
I think using schedule is probably right over here.
Correct me if i got it all wrong.
on x86:
kvm_arch_vcpu_ioctl_run
vcpu_run
for () {
.. run guest..
xfer_to_guest_mode_handle_work
schedule
}
on Powerpc: ( taking book3s_hv flavour):
kvm_arch_vcpu_ioctl_run
kvmppc_vcpu_run_hv *1
do while() {
kvmhv_run_single_vcpu or kvmppc_run_vcpu
-- checking for need_resched and signals and bails out
*2
}
*1 - checks for need resched and signals before entering guest
*2 - checks for need resched and signals while running the guest
This patch is addressing only *1 but it needs to address *2 as well using
generic framework.
I think it is doable for books3s_hv atleast. (though might need rewrite)
__kvmppc_vcpu_run is a block box to me yet. I think it first makes sense
to move it C and then try use the xfer_to_guest_mode_handle_work.
nick, vaibhav, any idea on __kvmppc_vcpu_run on how is it handling signal
pending, and need_resched.
So this is going to need more work specially on *2 and doing that is also key
for preempt=lazy/full to work
for kvm on powepc. will try to figure out.
