Re: [PATCH v6 9/9] KVM: VMX: Optimize memory allocation for PID-pointer table

[Chao Gao]

On Fri, Feb 25, 2022 at 07:29:39PM +0200, Maxim Levitsky wrote:
>On Fri, 2022-02-25 at 16:22 +0800, Zeng Guang wrote:
>> Current kvm allocates 8 pages in advance for Posted Interrupt Descriptor
>> pointer (PID-pointer) table to accommodate vCPUs with APIC ID up to
>> KVM_MAX_VCPU_IDS - 1. This policy wastes some memory because most of
>> VMs have less than 512 vCPUs and then just need one page.
>> 
>> If user hypervisor specify max practical vcpu id prior to vCPU creation,
>> IPIv can allocate only essential memory for PID-pointer table and reduce
>> the memory footprint of VMs.
>> 
>> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
>> Signed-off-by: Zeng Guang <guang.zeng@xxxxxxxxx>
>> ---
>>  arch/x86/kvm/vmx/vmx.c | 45 ++++++++++++++++++++++++++++--------------
>>  1 file changed, 30 insertions(+), 15 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 0cb141c277ef..22bfb4953289 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -230,9 +230,6 @@ static const struct {
>>  };
>>  
>>  #define L1D_CACHE_ORDER 4
>> -
>> -/* PID(Posted-Interrupt Descriptor)-pointer table entry is 64-bit long */
>> -#define MAX_PID_TABLE_ORDER get_order(KVM_MAX_VCPU_IDS * sizeof(u64))
>>  #define PID_TABLE_ENTRY_VALID 1
>>  
>>  static void *vmx_l1d_flush_pages;
>> @@ -4434,6 +4431,24 @@ static u32 vmx_secondary_exec_control(struct vcpu_vmx *vmx)
>>  	return exec_control;
>>  }
>>  
>> +static int vmx_alloc_pid_table(struct kvm_vmx *kvm_vmx)
>> +{
>> +	struct page *pages;
>> +
>> +	if(kvm_vmx->pid_table)
>> +		return 0;
>> +
>> +	pages = alloc_pages(GFP_KERNEL | __GFP_ZERO,
>> +			get_order(kvm_vmx->kvm.arch.max_vcpu_id * sizeof(u64)));
>> +
>> +	if (!pages)
>> +		return -ENOMEM;
>> +
>> +	kvm_vmx->pid_table = (void *)page_address(pages);
>> +	kvm_vmx->pid_last_index = kvm_vmx->kvm.arch.max_vcpu_id - 1;
>> +	return 0;
>> +}
>> +
>>  #define VMX_XSS_EXIT_BITMAP 0
>>  
>>  static void init_vmcs(struct vcpu_vmx *vmx)
>> @@ -7159,6 +7174,16 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
>>  			goto free_vmcs;
>>  	}
>>  
>> +	if (enable_ipiv && kvm_vcpu_apicv_active(vcpu)) {
>> +		struct kvm_vmx *kvm_vmx = to_kvm_vmx(vcpu->kvm);
>> +
>> +		mutex_lock(&vcpu->kvm->lock);
>> +		err = vmx_alloc_pid_table(kvm_vmx);
>> +		mutex_unlock(&vcpu->kvm->lock);
>> +		if (err)
>> +			goto free_vmcs;
>> +	}
>
>This could be dangerous. If APICv is temporary inhibited,
>this code won't run and we will end up without PID table.
>
>I think that kvm_vcpu_apicv_active should be just dropped
>from this condition.

Agreed. Will fix it.