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

Tang Chen
2014-07-14T09:16:07Z

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.
> .
>