Re: [PATCH 02/21] KVM: x86/tdp_mmu: Add a helper function to walk down the TDP MMU

[Paolo Bonzini]

On 9/4/24 05:07, Rick Edgecombe wrote:
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
>
> Export a function to walk down the TDP without modifying it and simply
> check if a PGA is mapped.
>
> Future changes will support pre-populating TDX private memory. In order to
> implement this KVM will need to check if a given GFN is already
> pre-populated in the mirrored EPT. [1]
>
> There is already a TDP MMU walker, kvm_tdp_mmu_get_walk() for use within
> the KVM MMU that almost does what is required. However, to make sense of
> the results, MMU internal PTE helpers are needed. Refactor the code to
> provide a helper that can be used outside of the KVM MMU code.
>
> Refactoring the KVM page fault handler to support this lookup usage was
> also considered, but it was an awkward fit.
>
> kvm_tdp_mmu_gpa_is_mapped() is based on a diff by Paolo Bonzini.
>
> Link: https://lore.kernel.org/kvm/ZfBkle1eZFfjPI8l@xxxxxxxxxx/ [1]
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
> TDX MMU part 2 v1:
>   - Change exported function to just return of GPA is mapped because "You
>     are executing with the filemap_invalidate_lock() taken, and therefore
>     cannot race with kvm_gmem_punch_hole()" (Paolo)
>     https://lore.kernel.org/kvm/CABgObfbpNN842noAe77WYvgi5MzK2SAA_FYw-=fGa+PcT_Z22w@xxxxxxxxxxxxxx/
>   - Take root hpa instead of enum (Paolo)
>
> TDX MMU Prep v2:
>   - Rename function with "mirror" and use root enum
>
> TDX MMU Prep:
>   - New patch
> ---
>   arch/x86/kvm/mmu.h         |  3 +++
>   arch/x86/kvm/mmu/mmu.c     |  3 +--
>   arch/x86/kvm/mmu/tdp_mmu.c | 37 ++++++++++++++++++++++++++++++++-----
>   3 files changed, 36 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 8f289222b353..5faa416ac874 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -254,6 +254,9 @@ extern bool tdp_mmu_enabled;
>   #define tdp_mmu_enabled false
>   #endif
>   
> +bool kvm_tdp_mmu_gpa_is_mapped(struct kvm_vcpu *vcpu, u64 gpa);
> +int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code, u8 *level);
> +
>   static inline bool kvm_memslots_have_rmaps(struct kvm *kvm)
>   {
>   	return !tdp_mmu_enabled || kvm_shadow_root_allocated(kvm);
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 7e66d7c426c1..01808cdf8627 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4713,8 +4713,7 @@ int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>   	return direct_page_fault(vcpu, fault);
>   }
>   
> -static int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code,
> -			    u8 *level)
> +int kvm_tdp_map_page(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code, u8 *level)
>   {
>   	int r;
>   
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 37b3769a5d32..019b43723d90 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1911,16 +1911,13 @@ bool kvm_tdp_mmu_write_protect_gfn(struct kvm *kvm,
>    *
>    * Must be called between kvm_tdp_mmu_walk_lockless_{begin,end}.
>    */
> -int kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
> -			 int *root_level)
> +static int __kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
> +				  struct kvm_mmu_page *root)
>   {
> -	struct kvm_mmu_page *root = root_to_sp(vcpu->arch.mmu->root.hpa);
>   	struct tdp_iter iter;
>   	gfn_t gfn = addr >> PAGE_SHIFT;
>   	int leaf = -1;
>   
> -	*root_level = vcpu->arch.mmu->root_role.level;
> -
>   	tdp_mmu_for_each_pte(iter, vcpu->kvm, root, gfn, gfn + 1) {
>   		leaf = iter.level;
>   		sptes[leaf] = iter.old_spte;
> @@ -1929,6 +1926,36 @@ int kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
>   	return leaf;
>   }
>   
> +int kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
> +			 int *root_level)
> +{
> +	struct kvm_mmu_page *root = root_to_sp(vcpu->arch.mmu->root.hpa);
> +	*root_level = vcpu->arch.mmu->root_role.level;
> +
> +	return __kvm_tdp_mmu_get_walk(vcpu, addr, sptes, root);
> +}
> +
> +bool kvm_tdp_mmu_gpa_is_mapped(struct kvm_vcpu *vcpu, u64 gpa)
> +{
> +	struct kvm *kvm = vcpu->kvm;
> +	bool is_direct = kvm_is_addr_direct(kvm, gpa);
> +	hpa_t root = is_direct ? vcpu->arch.mmu->root.hpa :
> +				 vcpu->arch.mmu->mirror_root_hpa;
> +	u64 sptes[PT64_ROOT_MAX_LEVEL + 1], spte;
> +	int leaf;
> +
> +	lockdep_assert_held(&kvm->mmu_lock);
> +	rcu_read_lock();
> +	leaf = __kvm_tdp_mmu_get_walk(vcpu, gpa, sptes, root_to_sp(root));
> +	rcu_read_unlock();
> +	if (leaf < 0)
> +		return false;
> +
> +	spte = sptes[leaf];
> +	return is_shadow_present_pte(spte) && is_last_spte(spte, leaf);
> +}
> +EXPORT_SYMBOL_GPL(kvm_tdp_mmu_gpa_is_mapped);
> +
>   /*
>    * Returns the last level spte pointer of the shadow page walk for the given
>    * gpa, and sets *spte to the spte value. This spte may be non-preset. If no

Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

I will take another look at the locking after I see some callers.

Paolo