linux-kernel - Re: [PATCH v5 05/30] KVM: Provide more information in kernel log if hardware enabling fails

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Message-ID: <Y0cZdmySTNEUo6Ji@google.com>
Date:   Wed, 12 Oct 2022 19:45:58 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     isaku.yamahata@...el.com
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        isaku.yamahata@...il.com, Kai Huang <kai.huang@...el.com>,
        Chao Gao <chao.gao@...el.com>,
        Atish Patra <atishp@...shpatra.org>,
        Shaokun Zhang <zhangshaokun@...ilicon.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Huang Ying <ying.huang@...el.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v5 05/30] KVM: Provide more information in kernel log if
 hardware enabling fails

On Thu, Sep 22, 2022, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> Provide the name of the calling function to hardware_enable_nolock() and
> include it in the error message to provide additional information on
> exactly what path failed.

Changelog doesn't match the code.

> Opportunistically bump the pr_info() to pr_warn(), failure to enable
> virtualization support is warn-worthy as _something_ is wrong with the
> system.
> 
> Suggested-by: Chao Gao <chao.gao@...el.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
>  virt/kvm/kvm_main.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 4243a9541543..4f19c47aab1c 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -5006,7 +5006,8 @@ static void hardware_enable_nolock(void *junk)
>  	if (r) {
>  		cpumask_clear_cpu(cpu, cpus_hardware_enabled);
>  		atomic_inc(&hardware_enable_failed);
> -		pr_info("kvm: enabling virtualization on CPU%d failed\n", cpu);
> +		pr_warn("kvm: enabling virtualization on CPU%d failed during %pSb\n",
> +			cpu, __builtin_return_address(0));

I vote to drop this patch.  Trying to capture the caller is just a poor man's
version of WARN, and at the end of this series KVM should be at a point where KVM
can WARN when hardware enabling indicates a potentially fatal issue.

Specifically, kvm_arch_add_vm() shouldn't WARN since x86 can fail due a misbehaving
userspace.  kvm_arch_online_cpu() on the other hand can and should WARN since
failure in that case means hardware enabling succeeded on other CPUs, and in the x86
case, that KVM is actively running VMs.