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

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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@xxxxxxxxxxxxxx>
---
  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.
.

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