Re: [PATCH 03/16] KVM: x86/tdp_mmu: Add a helper function to walk down the TDP MMU

[Chao Gao]

On Tue, May 14, 2024 at 05:59:39PM -0700, Rick Edgecombe wrote:
>From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
>
>Export a function to walk down the TDP without modifying it.
>
>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, and verify the populated private memory
>PFN matches the current one.[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.
>
>Link: https://lore.kernel.org/kvm/ZfBkle1eZFfjPI8l@xxxxxxxxxx/ [1]
>Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
>Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
>---
>This helper will be used in the future change that implements
>KVM_TDX_INIT_MEM_REGION. Please refer to the following commit for the
>usage:
>https://github.com/intel/tdx/commit/2832c6d87a4e6a46828b193173550e80b31240d4
>
>TDX MMU Part 1:
> - New patch
>---
> arch/x86/kvm/mmu.h         |  3 +++
> arch/x86/kvm/mmu/tdp_mmu.c | 37 +++++++++++++++++++++++++++++++++----
> 2 files changed, 36 insertions(+), 4 deletions(-)
>
>diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
>index dc80e72e4848..3c7a88400cbb 100644
>--- a/arch/x86/kvm/mmu.h
>+++ b/arch/x86/kvm/mmu.h
>@@ -275,6 +275,9 @@ extern bool tdp_mmu_enabled;
> #define tdp_mmu_enabled false
> #endif
> 
>+int kvm_tdp_mmu_get_walk_private_pfn(struct kvm_vcpu *vcpu, u64 gpa,
>+				     kvm_pfn_t *pfn);
>+
> 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/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
>index 1259dd63defc..1086e3b2aa5c 100644
>--- a/arch/x86/kvm/mmu/tdp_mmu.c
>+++ b/arch/x86/kvm/mmu/tdp_mmu.c
>@@ -1772,16 +1772,14 @@ 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,
>+				  bool is_private)

is_private isn't used.

> {
> 	struct tdp_iter iter;
> 	struct kvm_mmu *mmu = vcpu->arch.mmu;
> 	gfn_t gfn = addr >> PAGE_SHIFT;
> 	int leaf = -1;
> 
>-	*root_level = vcpu->arch.mmu->root_role.level;
>-
> 	tdp_mmu_for_each_pte(iter, mmu, gfn, gfn + 1) {
> 		leaf = iter.level;
> 		sptes[leaf] = iter.old_spte;
>@@ -1790,6 +1788,37 @@ 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)
>+{
>+	*root_level = vcpu->arch.mmu->root_role.level;
>+
>+	return __kvm_tdp_mmu_get_walk(vcpu, addr, sptes, false);
>+}
>+
>+int kvm_tdp_mmu_get_walk_private_pfn(struct kvm_vcpu *vcpu, u64 gpa,
>+				     kvm_pfn_t *pfn)

private_pfn probably is a misnomer. shared/private is an attribute of
GPA rather than pfn. Since the function is to get pfn from gpa, how about
kvm_tdp_mmu_gpa_to_pfn()?

And the function is limited to handle private gpa only. It is an artificial
limitation we can get rid of easily. e.g., by making the function take
"is_private" boolean and relay it to __kvm_tdp_mmu_get_walk(). I know TDX
just calls the function to convert private gpa but having a generic API
can accommodate future use cases (e.g., get hpa from shared gpa) w/o the
need of refactoring.

>+{
>+	u64 sptes[PT64_ROOT_MAX_LEVEL + 1], spte;
>+	int leaf;
>+
>+	lockdep_assert_held(&vcpu->kvm->mmu_lock);
>+
>+	rcu_read_lock();
>+	leaf = __kvm_tdp_mmu_get_walk(vcpu, gpa, sptes, true);
>+	rcu_read_unlock();
>+	if (leaf < 0)
>+		return -ENOENT;
>+
>+	spte = sptes[leaf];
>+	if (!(is_shadow_present_pte(spte) && is_last_spte(spte, leaf)))
>+		return -ENOENT;
>+
>+	*pfn = spte_to_pfn(spte);
>+	return leaf;
>+}
>+EXPORT_SYMBOL_GPL(kvm_tdp_mmu_get_walk_private_pfn);
>+
> /*
>  * 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
>-- 
>2.34.1
>
>