linux-kernel - Re: [PATCH v4] KVM: Check the allocation of pv cpu mask

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Date:   Mon, 19 Oct 2020 20:36:22 +0800
From:   Haiwei Li <lihaiwei.kernel@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     pbonzini@...hat.com, sean.j.christopherson@...el.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        Haiwei Li <lihaiwei@...cent.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] KVM: Check the allocation of pv cpu mask

On 20/10/19 19:23, Vitaly Kuznetsov wrote:
> lihaiwei.kernel@...il.com writes:
> 
>> From: Haiwei Li <lihaiwei@...cent.com>
>>
>> check the allocation of per-cpu __pv_cpu_mask. Init
>> 'send_IPI_mask_allbutself' only when successful and check the allocation
>> of __pv_cpu_mask in 'kvm_flush_tlb_others'.
>>
>> Suggested-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> Signed-off-by: Haiwei Li <lihaiwei@...cent.com>
>> ---
>> v1 -> v2:
>>   * add CONFIG_SMP for kvm_send_ipi_mask_allbutself to prevent build error
>> v2 -> v3:
>>   * always check the allocation of __pv_cpu_mask in kvm_flush_tlb_others
>> v3 -> v4:
>>   * mov kvm_setup_pv_ipi to kvm_alloc_cpumask and get rid of kvm_apic_init
>>
>>   arch/x86/kernel/kvm.c | 53 +++++++++++++++++++++++++++++--------------
>>   1 file changed, 36 insertions(+), 17 deletions(-)
>>
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index 42c6e0deff9e..be28203cc098 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -547,16 +547,6 @@ static void kvm_send_ipi_mask_allbutself(const struct cpumask *mask, int vector)
>>   	__send_ipi_mask(local_mask, vector);
>>   }
>>   
>> -/*
>> - * Set the IPI entry points
>> - */
>> -static void kvm_setup_pv_ipi(void)
>> -{
>> -	apic->send_IPI_mask = kvm_send_ipi_mask;
>> -	apic->send_IPI_mask_allbutself = kvm_send_ipi_mask_allbutself;
>> -	pr_info("setup PV IPIs\n");
>> -}
>> -
>>   static void kvm_smp_send_call_func_ipi(const struct cpumask *mask)
>>   {
>>   	int cpu;
>> @@ -619,6 +609,11 @@ static void kvm_flush_tlb_others(const struct cpumask *cpumask,
>>   	struct kvm_steal_time *src;
>>   	struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);
>>   
>> +	if (unlikely(!flushmask)) {
>> +		native_flush_tlb_others(cpumask, info);
>> +		return;
>> +	}
>> +
>>   	cpumask_copy(flushmask, cpumask);
>>   	/*
>>   	 * We have to call flush only on online vCPUs. And
>> @@ -732,10 +727,6 @@ static uint32_t __init kvm_detect(void)
>>   
>>   static void __init kvm_apic_init(void)
>>   {
>> -#if defined(CONFIG_SMP)
>> -	if (pv_ipi_supported())
>> -		kvm_setup_pv_ipi();
>> -#endif
>>   }
> 
> Do we still need the now-empty function?

It's not necessary. I will remove it.

> 
>>   
>>   static void __init kvm_init_platform(void)
>> @@ -765,10 +756,18 @@ static __init int activate_jump_labels(void)
>>   }
>>   arch_initcall(activate_jump_labels);
>>   
>> +static void kvm_free_cpumask(void)
>> +{
>> +	unsigned int cpu;
>> +
>> +	for_each_possible_cpu(cpu)
>> +		free_cpumask_var(per_cpu(__pv_cpu_mask, cpu));
>> +}
>> +
>>   static __init int kvm_alloc_cpumask(void)
>>   {
>>   	int cpu;
>> -	bool alloc = false;
>> +	bool alloc = false, alloced = true;
>>   
>>   	if (!kvm_para_available() || nopv)
>>   		return 0;
>> @@ -783,10 +782,30 @@ static __init int kvm_alloc_cpumask(void)
>>   
>>   	if (alloc)
>>   		for_each_possible_cpu(cpu) {
>> -			zalloc_cpumask_var_node(per_cpu_ptr(&__pv_cpu_mask, cpu),
>> -				GFP_KERNEL, cpu_to_node(cpu));
>> +			if (!zalloc_cpumask_var_node(
>> +				per_cpu_ptr(&__pv_cpu_mask, cpu),
>> +				GFP_KERNEL, cpu_to_node(cpu))) {
>> +				alloced = false;
>> +				break;
>> +			}
>>   		}
>>   
>> +#if defined(CONFIG_SMP)
>> +	/* Set the IPI entry points */
>> +	if (pv_ipi_supported()) {
> 
> What if we define pv_ipi_supported() in !CONFIG_SMP case as 'false'?
> 
> The code we have above:
> 
>          if (pv_tlb_flush_supported())
> 		alloc = true;
> 
> #if defined(CONFIG_SMP)
>          if (pv_ipi_supported())
> 		alloc = true;
> #endif
> 
>        	if (alloc)
> ...
> 
> will transform into 'if (pv_tlb_flush_supported() ||
> pv_ipi_supported())' and we'll get rid of 'alloc' variable.
> 
> Also, we can probably get rid of this new 'alloced' variable and switch
> to checking if the cpumask for the last CPU in cpu_possible_mask is not
> NULL.

Get it. It's a good point. I will do it. Thanks for your patience and 
kindness.

>   
>> +		apic->send_IPI_mask = kvm_send_ipi_mask;
>> +		if (alloced)
>> +			apic->send_IPI_mask_allbutself =
>> +				kvm_send_ipi_mask_allbutself;
>> +		pr_info("setup PV IPIs\n");
> 
> I'd rather not set 'apic->send_IPI_mask = kvm_send_ipi_mask' in case we
> failed to alloc cpumask too. It is weird that in case of an allocation
> failure *some* IPIs will use the PV path and some won't. It's going to
> be a nightmare to debug.

Agree. And 'pv_ops.mmu.tlb_remove_table = tlb_remove_table' should not 
be set either. What do you think? Thanks.

     Haiwei Li