Re: [RESEND PATCH v2 4/5] kvm: Remove ept_identity_pagetable from struct kvm_arch.

[Tang Chen <tangchen@cn.fujitsu.com>]

Hi Gleb,

Please see below.

On 07/12/2014 03:44 PM, Gleb Natapov wrote:
> On Wed, Jul 09, 2014 at 10:08:03AM +0800, Tang Chen wrote:
>> kvm_arch->ept_identity_pagetable holds the ept identity pagetable page. But
>> it is never used to refer to the page at all.
>>
>> In vcpu initialization, it indicates two things:
>> 1. indicates if ept page is allocated
>> 2. indicates if a memory slot for identity page is initialized
>>
>> Actually, kvm_arch->ept_identity_pagetable_done is enough to tell if the ept
>> identity pagetable is initialized. So we can remove ept_identity_pagetable.
>>
>> Signed-off-by: Tang Chen<tangchen@cn.fujitsu.com>
>> ---
>>   arch/x86/include/asm/kvm_host.h |  1 -
>>   arch/x86/kvm/vmx.c              | 25 +++++++++++--------------
>>   2 files changed, 11 insertions(+), 15 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 4931415..62f973e 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -578,7 +578,6 @@ struct kvm_arch {
>>
>>   	gpa_t wall_clock;
>>
>> -	struct page *ept_identity_pagetable;
>>   	bool ept_identity_pagetable_done;
>>   	gpa_t ept_identity_map_addr;
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 0918635e..fe2e5f4 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -741,6 +741,7 @@ static void vmx_sync_pir_to_irr_dummy(struct kvm_vcpu *vcpu);
>>   static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx);
>>   static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx);
>>   static bool vmx_mpx_supported(void);
>> +static int alloc_identity_pagetable(struct kvm *kvm);
>>
>>   static DEFINE_PER_CPU(struct vmcs *, vmxarea);
>>   static DEFINE_PER_CPU(struct vmcs *, current_vmcs);
>> @@ -3921,21 +3922,21 @@ out:
>>
>>   static int init_rmode_identity_map(struct kvm *kvm)
>>   {
>> -	int i, idx, r, ret;
>> +	int i, idx, r, ret = 0;
>>   	pfn_t identity_map_pfn;
>>   	u32 tmp;
>>
>>   	if (!enable_ept)
>>   		return 1;
>> -	if (unlikely(!kvm->arch.ept_identity_pagetable)) {
>> -		printk(KERN_ERR "EPT: identity-mapping pagetable "
>> -			"haven't been allocated!\n");
>> -		return 0;
>> -	}
>>   	if (likely(kvm->arch.ept_identity_pagetable_done))
>>   		return 1;
>> -	ret = 0;
>>   	identity_map_pfn = kvm->arch.ept_identity_map_addr>>  PAGE_SHIFT;
>> +
>> +	mutex_lock(&kvm->slots_lock);
> Why move this out of alloc_identity_pagetable()?
>

Referring to the original code, I think mutex_lock(&kvm->slots_lock) is 
used
to protect kvm->arch.ept_identity_pagetable. If two or more threads try to
modify it at the same time, the mutex ensures that the identity table is 
only
allocated once.

Now, we dropped kvm->arch.ept_identity_pagetable. And use 
kvm->arch.ept_identity_pagetable_done
to check if the identity table is allocated and initialized. So we 
should protect
memory slot operation in alloc_identity_pagetable() and 
kvm->arch.ept_identity_pagetable_done
with this mutex.

Of course, I can see that the name "slots_lock" indicates that it may be 
used
to protect the memory slot operation only. Maybe move it out here is not 
suitable.

If I'm wrong, please tell me.

>> +	r = alloc_identity_pagetable(kvm);
>> +	if (r)
>> +		goto out2;
>> +
>>   	idx = srcu_read_lock(&kvm->srcu);
>>   	r = kvm_clear_guest_page(kvm, identity_map_pfn, 0, PAGE_SIZE);
>>   	if (r<  0)
>> @@ -3953,6 +3954,9 @@ static int init_rmode_identity_map(struct kvm *kvm)
>>   	ret = 1;
>>   out:
>>   	srcu_read_unlock(&kvm->srcu, idx);
>> +
>> +out2:
>> +	mutex_unlock(&kvm->slots_lock);
>>   	return ret;
>>   }
>>
>> @@ -4006,9 +4010,6 @@ static int alloc_identity_pagetable(struct kvm *kvm)
>>   	struct kvm_userspace_memory_region kvm_userspace_mem;
>>   	int r = 0;
>>
>> -	mutex_lock(&kvm->slots_lock);
>> -	if (kvm->arch.ept_identity_pagetable)
>> -		goto out;
>>   	kvm_userspace_mem.slot = IDENTITY_PAGETABLE_PRIVATE_MEMSLOT;
>>   	kvm_userspace_mem.flags = 0;
>>   	kvm_userspace_mem.guest_phys_addr =
>> @@ -4025,9 +4026,7 @@ static int alloc_identity_pagetable(struct kvm *kvm)
>>   		goto out;
>>   	}
>>
>> -	kvm->arch.ept_identity_pagetable = page;
> I think we can drop gfn_to_page() above too now. Why would we need it?
>

Yes, will remove it in the next version.

Thanks.

>>   out:
>> -	mutex_unlock(&kvm->slots_lock);
>>   	return r;
>>   }
>>
>> @@ -7583,8 +7582,6 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id)
>>   			kvm->arch.ept_identity_map_addr =
>>   				VMX_EPT_IDENTITY_PAGETABLE_ADDR;
>>   		err = -ENOMEM;
>> -		if (alloc_identity_pagetable(kvm) != 0)
>> -			goto free_vmcs;
>>   		if (!init_rmode_identity_map(kvm))
>>   			goto free_vmcs;
>>   	}
>> --
>> 1.8.3.1
>>
>
> --
> 			Gleb.
> .
>