Re: [PATCH 4/8] KVM: PPC: Book3S HV: Ratelimit machine check messages coming from guests

[Michael Ellerman <mpe@ellerman.id.au>]

Nicholas Piggin <npiggin@gmail.com> writes:
> A number of machine check exceptions are triggerable by the guest.
> Ratelimit these to avoid a guest flooding the host console and logs.
>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  arch/powerpc/kvm/book3s_hv.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index e3b1839fc251..c94f9595133d 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -1328,8 +1328,12 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
>  		r = RESUME_GUEST;
>  		break;
>  	case BOOK3S_INTERRUPT_MACHINE_CHECK:
> -		/* Print the MCE event to host console. */
> -		machine_check_print_event_info(&vcpu->arch.mce_evt, false, true);
> +		/*
> +		 * Print the MCE event to host console. Ratelimit so the guest
> +		 * can't flood the host log.
> +		 */
> +		if (printk_ratelimit())
> +			machine_check_print_event_info(&vcpu->arch.mce_evt,false, true);

You're not supposed to use printk_ratelimit(), because there's a single
rate limit state for all printks. ie. some other noisty printk() can
cause this one to never be printed.

I folded this in:

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index cbbc4f0a26fe..cfaa91b27112 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -1327,12 +1327,14 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
 	case BOOK3S_INTERRUPT_SYSTEM_RESET:
 		r = RESUME_GUEST;
 		break;
-	case BOOK3S_INTERRUPT_MACHINE_CHECK:
+	case BOOK3S_INTERRUPT_MACHINE_CHECK: {
+		static DEFINE_RATELIMIT_STATE(rs, DEFAULT_RATELIMIT_INTERVAL,
+					      DEFAULT_RATELIMIT_BURST);
 		/*
 		 * Print the MCE event to host console. Ratelimit so the guest
 		 * can't flood the host log.
 		 */
-		if (printk_ratelimit())
+		if (__ratelimit(&rs))
 			machine_check_print_event_info(&vcpu->arch.mce_evt,false, true);
 
 		/*
@@ -1361,6 +1363,7 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
 
 		r = RESUME_HOST;
 		break;
+	}
 	case BOOK3S_INTERRUPT_PROGRAM:
 	{
 		ulong flags;
@@ -1520,12 +1523,16 @@ static int kvmppc_handle_nested_exit(struct kvm_vcpu *vcpu)
 		r = RESUME_GUEST;
 		break;
 	case BOOK3S_INTERRUPT_MACHINE_CHECK:
+	{
+		static DEFINE_RATELIMIT_STATE(rs, DEFAULT_RATELIMIT_INTERVAL,
+					      DEFAULT_RATELIMIT_BURST);
 		/* Pass the machine check to the L1 guest */
 		r = RESUME_HOST;
 		/* Print the MCE event to host console. */
-		if (printk_ratelimit())
+		if (__ratelimit(&rs))
 			machine_check_print_event_info(&vcpu->arch.mce_evt, false, true);
 		break;
+	}
 	/*
 	 * We get these next two if the guest accesses a page which it thinks
 	 * it has mapped but which is not actually present, either because


cheers