linux-kernel - Re: [PATCH v2 3/4] KVM: Rename and move CPUHP_AP_KVM

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Date:   Wed, 9 Feb 2022 00:29:57 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Gao <chao.gao@...el.com>
Cc:     kvm@...r.kernel.org, pbonzini@...hat.com, kevin.tian@...el.com,
        tglx@...utronix.de, John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Shaokun Zhang <zhangshaokun@...ilicon.com>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        Tony Lindgren <tony@...mide.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/4] KVM: Rename and move CPUHP_AP_KVM_STARTING to
 ONLINE section

On Tue, Jan 18, 2022, Chao Gao wrote:
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 148f7169b431..528741601122 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4856,13 +4856,25 @@ static void hardware_enable_nolock(void *junk)
>  	}
>  }
>  
> -static int kvm_starting_cpu(unsigned int cpu)
> +static int kvm_online_cpu(unsigned int cpu)
>  {
> +	int ret = 0;
> +
>  	raw_spin_lock(&kvm_count_lock);
> -	if (kvm_usage_count)
> +	/*
> +	 * Abort the CPU online process if hardware virtualization cannot
> +	 * be enabled. Otherwise running VMs would encounter unrecoverable
> +	 * errors when scheduled to this CPU.
> +	 */
> +	if (kvm_usage_count) {


>  		hardware_enable_nolock(NULL);
> +		if (atomic_read(&hardware_enable_failed)) {

This needs:

		atomic_set(&hardware_enable_failed, 0);

otherwise failure to online one CPU will prevent onlining other non-broken CPUs.
It's probably worth adding a WARN_ON_ONCE above this too, e.g.

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 70e034cbe813..b25a00c76b3a 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4863,8 +4863,11 @@ static int kvm_online_cpu(unsigned int cpu)
         * errors when scheduled to this CPU.
         */
        if (kvm_usage_count) {
+               WARN_ON_ONCE(atomic_read(&hardware_enable_failed));
+
                hardware_enable_nolock(NULL);
                if (atomic_read(&hardware_enable_failed)) {
+                       atomic_set(&hardware_enable_failed, 0);
                        ret = -EIO;
                        pr_warn("kvm: abort onlining CPU%d", cpu);
                }


> +			ret = -EIO;
> +			pr_warn("kvm: abort onlining CPU%d", cpu);

This is somewhat redundant with the pr_info() message in hardware_enable_nolock().
What about adding the below as a prep patch?  I think/hope it would be obvious to
the user/admin that onlining the CPU failed?  E.g. this for the output

  kvm: enabling virtualization on CPU2 failed during hardware_enable_all()

From: Sean Christopherson <seanjc@...gle.com>
Date: Tue, 8 Feb 2022 13:26:19 -0800
Subject: [PATCH] KVM: Provide more information in kernel log if hardware
 enabling fails

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.

Opportunistically bump the pr_info() to pr_warn(), failure to enable
virtualization support is warn-worthy as _something_ is wrong with the
system.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 virt/kvm/kvm_main.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index be614a6325e4..23481fd746aa 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4833,7 +4833,7 @@ static struct miscdevice kvm_dev = {
 	&kvm_chardev_ops,
 };

-static void hardware_enable_nolock(void *junk)
+static void hardware_enable_nolock(void *caller_name)
 {
 	int cpu = raw_smp_processor_id();
 	int r;
@@ -4848,7 +4848,8 @@ 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 %s()\n",
+			cpu, (const char *)caller_name);
 	}
 }

@@ -4856,7 +4857,7 @@ static int kvm_starting_cpu(unsigned int cpu)
 {
 	raw_spin_lock(&kvm_count_lock);
 	if (kvm_usage_count)
-		hardware_enable_nolock(NULL);
+		hardware_enable_nolock((void *)__func__);
 	raw_spin_unlock(&kvm_count_lock);
 	return 0;
 }
@@ -4905,7 +4906,7 @@ static int hardware_enable_all(void)
 	kvm_usage_count++;
 	if (kvm_usage_count == 1) {
 		atomic_set(&hardware_enable_failed, 0);
-		on_each_cpu(hardware_enable_nolock, NULL, 1);
+		on_each_cpu(hardware_enable_nolock, (void *)__func__, 1);

 		if (atomic_read(&hardware_enable_failed)) {
 			hardware_disable_all_nolock();
@@ -5530,7 +5531,7 @@ static void kvm_resume(void)
 #ifdef CONFIG_LOCKDEP
 		WARN_ON(lockdep_is_held(&kvm_count_lock));
 #endif
-		hardware_enable_nolock(NULL);
+		hardware_enable_nolock((void *)__func__);
 	}
 }


base-commit: 357ef9d9c0728bc2bbb9810c662263bba6b8dbc7
--