Re: [PATCH v5 2/7] kvm: Remove ept_identity_pagetable from struct kvm_arch.

Paolo Bonzini
2014-09-11T09:14:40Z

Il 11/09/2014 07:38, Tang Chen ha scritto:
> 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.
>
> NOTE: In the original code, ept identity pagetable page is pinned in memroy.
> As a result, it cannot be migrated/hot-removed. After this patch, since
> kvm_arch->ept_identity_pagetable is removed, ept identity pagetable page
> is no longer pinned in memory. And it can be migrated/hot-removed.
>
> Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
> Reviewed-by: Gleb Natapov <gleb@kernel.org>
> ---
> arch/x86/include/asm/kvm_host.h | 1 -
> arch/x86/kvm/vmx.c | 50 ++++++++++++++++++++---------------------
> arch/x86/kvm/x86.c | 2 --
> 3 files changed, 25 insertions(+), 28 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 7c492ed..35171c7 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -580,7 +580,6 @@ 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 4b80ead..953d529 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -743,6 +743,7 @@ static u32 vmx_segment_access_rights(struct kvm_segment *var);
> 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 int alloc_identity_pagetable(struct kvm *kvm);
>
> static DEFINE_PER_CPU(struct vmcs *, vmxarea);
> static DEFINE_PER_CPU(struct vmcs *, current_vmcs);
> @@ -3938,21 +3939,27 @@ 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;
> +
> + /* Protect kvm->arch.ept_identity_pagetable_done. */
> + mutex_lock(&kvm->slots_lock);
> +
> + if (likely(kvm->arch.ept_identity_pagetable_done)) {
> + ret = 1;
> + goto out2;
> }
> - if (likely(kvm->arch.ept_identity_pagetable_done))
> - return 1;
> - ret = 0;
> +
> identity_map_pfn = kvm->arch.ept_identity_map_addr >> PAGE_SHIFT;
> +
> + 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)
> @@ -3970,6 +3977,9 @@ 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;
> }
>
> @@ -4019,31 +4029,23 @@ out:
>
> static int alloc_identity_pagetable(struct kvm *kvm)
> {
> - struct page *page;
> + /*
> + * In init_rmode_identity_map(), kvm->arch.ept_identity_pagetable_done
> + * is checked before calling this function and set to true after the
> + * calling. The access to kvm->arch.ept_identity_pagetable_done should
> + * be protected by kvm->slots_lock.
> + */

Not just the access, also the call to this function must be protected by
the lock.

So replace this commend by just

/* Called with kvm->slots_lock held. */

and add a BUG_ON that ept_identity_pagetable_done is false.

Paolo

> 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 =
> kvm->arch.ept_identity_map_addr;
> kvm_userspace_mem.memory_size = PAGE_SIZE;
> r = __kvm_set_memory_region(kvm, &kvm_userspace_mem);
> - if (r)
> - goto out;
>
> - page = gfn_to_page(kvm, kvm->arch.ept_identity_map_addr >> PAGE_SHIFT);
> - if (is_error_page(page)) {
> - r = -EFAULT;
> - goto out;
> - }
> -
> - kvm->arch.ept_identity_pagetable = page;
> -out:
> - mutex_unlock(&kvm->slots_lock);
> return r;
> }
>
> @@ -7643,8 +7645,6 @@ 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;
> }
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8f1e22d..e05bd58 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7239,8 +7239,6 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
> kvm_free_vcpus(kvm);
> if (kvm->arch.apic_access_page)
> put_page(kvm->arch.apic_access_page);
> - if (kvm->arch.ept_identity_pagetable)
> - put_page(kvm->arch.ept_identity_pagetable);
> kfree(rcu_dereference_check(kvm->arch.apic_map, 1));
> }
>
>