linux-kernel - Re: [PATCH] KVM: arm64: Ratelimit error log during guest debug exception

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Message-ID: <87sfz4qx9r.wl-maz@kernel.org>
Date:   Fri, 20 Aug 2021 10:29:52 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
Cc:     Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Peter Shier <pshier@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Reiji Watanabe <reijiw@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: Ratelimit error log during guest debug exception

On Thu, 19 Aug 2021 23:34:06 +0100,
Raghavendra Rao Ananta <rananta@...gle.com> wrote:
> 
> Potentially, the guests could trigger a debug exception that's
> outside the exception class range.

How? All the exception classes that lead to this functions are already
handled in the switch/case statement.

> This could lead to an excessive syslog flooding. Hence, ratelimit
> the error message.
> 
> Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>
> ---
>  arch/arm64/kvm/handle_exit.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 04ebab299aa4..c7cec7ffe93c 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -134,7 +134,7 @@ static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu)
>  	case ESR_ELx_EC_BRK64:
>  		break;
>  	default:
> -		kvm_err("%s: un-handled case esr: %#08x\n",
> +		kvm_pr_unimpl("%s: un-handled case esr: %#08x\n",
>  			__func__, (unsigned int) esr);
>  		ret = -1;
>  		break;

My take on this is that this code isn't reachable, and that it could
be better rewritten as:

diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 6f48336b1d86..ae7ec086827b 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -119,28 +119,14 @@ static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu)
 {
 	struct kvm_run *run = vcpu->run;
 	u32 esr = kvm_vcpu_get_esr(vcpu);
-	int ret = 0;
 
 	run->exit_reason = KVM_EXIT_DEBUG;
 	run->debug.arch.hsr = esr;
 
-	switch (ESR_ELx_EC(esr)) {
-	case ESR_ELx_EC_WATCHPT_LOW:
+	if (ESR_ELx_EC(esr) ==  ESR_ELx_EC_WATCHPT_LOW)
 		run->debug.arch.far = vcpu->arch.fault.far_el2;
-		fallthrough;
-	case ESR_ELx_EC_SOFTSTP_LOW:
-	case ESR_ELx_EC_BREAKPT_LOW:
-	case ESR_ELx_EC_BKPT32:
-	case ESR_ELx_EC_BRK64:
-		break;
-	default:
-		kvm_err("%s: un-handled case esr: %#08x\n",
-			__func__, (unsigned int) esr);
-		ret = -1;
-		break;
-	}
 
-	return ret;
+	return 0;
 }
 
 static int kvm_handle_unknown_ec(struct kvm_vcpu *vcpu)

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.