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

Hi Narayana.

On 10/1/26 4:56 PM, Narayana Murty N wrote:

Hi Shrikanth,
Thanks for the fixes. I had one question on patch 1.

See response below.

quoted

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();

Would it be simpler to move kvmppc_fix_ee_before_entry() after the
FP/Altivec loads instead?

The normal kvmppc_vcpu_run() entry path already loads the guest
FP/Altivec state while interrupts are still disabled and calls
kvmppc_fix_ee_before_entry() immediately before entering the guest.

So could this path follow the same ordering:

kvmppc_load_guest_fp(vcpu);
kvmppc_load_guest_altivec(vcpu);
kvmppc_fix_ee_before_entry();

That would avoid making the software IRQ state enabled before loading
the guest FP/Altivec state, and also avoid the additional
preempt_disable()/preempt_enable_no_resched() pair.

Thanks,
Narayana Murty.

quoted

          }
      }

I thought I had put that for discussion after ---, but looks like I forgot.

I don't mind the above too.  but I didn't have a way to test it.
So kept it as is based on what Christian said works for him.

If you have a way to test the patches, please let me know.
We can try that too.