[PATCH 1/2] KVM: PPC: BookE: Disable preemption before loading guest FP and Altivec

Christian reported that booting preemptible kernel on FSL Cyrus+ board
causes boot hang.

The logs pointed that system was busy in printing below warning.

WARNING: at .enable_kernel_fp+0x30/0x78, CPU#3: qemu-system-ppc/4884
Modules linked in:
CPU: 3 UID: 1000 PID: 4884 Comm: qemu-system-ppc Not tainted 7.3.0-rc1-powerpc64-smp-preempt #1 PREEMPT
NIP [c000000000003338] .enable_kernel_fp+0x30/0x78
LR [c00000000005de84] .kvmppc_load_guest_fp+0x30/0x80
Call Trace:
[c000000085ca7700] [c00000000005de84] .kvmppc_load_guest_fp+0x30/0x80
[c000000085ca7780] [c00000000005f2a0] .kvmppc_handle_exit+0x5bc/0x5cc
[c000000085ca7830] [c00000000006204c] .kvmppc_resume_host+0xb8/0x10c

Which is...

void enable_kernel_fp(void)
{
        unsigned long cpumsr;
        WARN_ON(preemptible());

And...

Though irq's are hard disabled after kvmppc_prepare_to_enter, but
kvmppc_fix_ee_before_entry enables the softmask's IRQ state.
That causes the irqs_disabled to return false.
Hence leading to the warnings.

Fix it by disabling the preemption using the preempt disable.
Note, it is calling noresched variant of preempt enable, since hard
irq are disabled. It is likely not a good idea to call schedule.

Fixes: 3efc7da61f6c ("KVM: PPC: Book3E: Increase FPU laziness")
Reported-by: Christian Zigotzky <redacted>
Closes: https://lore.kernel.org/all/33342fbf-eb7b-bde6-2c8c-254fe8bfb993@xenosoft.de/ (local)
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 arch/powerpc/kvm/booke.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c
index 13ad4cf5fa71..5b9118eefe1d 100644
--- a/arch/powerpc/kvm/booke.c
+++ b/arch/powerpc/kvm/booke.c

@@ -1404,10 +1404,17 @@ int kvmppc_handle_exit(struct kvm_vcpu *vcpu, unsigned int exit_nr)
 		if (s <= 0)
 			r = (s << 2) | RESUME_HOST | (r & RESUME_FLAG_NV);
 		else {
-			/* interrupts now hard-disabled */
+			/*
+			 * kvmppc_fix_ee_before_entry() marks the software
+			 * IRQ state enabled while interrupts are still
+			 * hard-disabled. So disable preemption while loading
+			 * guest FP and Altivec.
+			 */
 			kvmppc_fix_ee_before_entry();
+			preempt_disable();
 			kvmppc_load_guest_fp(vcpu);
 			kvmppc_load_guest_altivec(vcpu);
+			preempt_enable_no_resched();
 		}
 	}
 

-- 
2.52.0