linux-kernel - [RFC PATCH 01/12] KVM: TDX: Drop PROVE_MMU=y sanity check on to-be-populated mappings

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Message-ID: <20250827000522.4022426-2-seanjc@google.com>
Date: Tue, 26 Aug 2025 17:05:11 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Michael Roth <michael.roth@....com>, Yan Zhao <yan.y.zhao@...el.com>, 
	Ira Weiny <ira.weiny@...el.com>, Vishal Annapurve <vannapurve@...gle.com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: [RFC PATCH 01/12] KVM: TDX: Drop PROVE_MMU=y sanity check on
 to-be-populated mappings

Drop TDX's sanity check that an S-EPT mapping isn't zapped between creating
said mapping and doing TDH.MEM.PAGE.ADD, as the check is simultaneously
superfluous and incomplete.  Per commit 2608f1057601 ("KVM: x86/tdp_mmu:
Add a helper function to walk down the TDP MMU"), the justification for
introducing kvm_tdp_mmu_gpa_is_mapped() was to check that the target gfn
was pre-populated, with a link that points to this snippet:

 : > One small question:
 : >
 : > What if the memory region passed to KVM_TDX_INIT_MEM_REGION hasn't been pre-
 : > populated?  If we want to make KVM_TDX_INIT_MEM_REGION work with these regions,
 : > then we still need to do the real map.  Or we can make KVM_TDX_INIT_MEM_REGION
 : > return error when it finds the region hasn't been pre-populated?
 :
 : Return an error.  I don't love the idea of bleeding so many TDX details into
 : userspace, but I'm pretty sure that ship sailed a long, long time ago.

But that justification makes little sense for the final code, as simply
doing TDH.MEM.PAGE.ADD without a paranoid sanity check will return an error
if the S-EPT mapping is invalid (as evidenced by the code being guarded
with CONFIG_KVM_PROVE_MMU=y).

The sanity check is also incomplete in the sense that mmu_lock is dropped
between the check and TDH.MEM.PAGE.ADD, i.e. will only detect KVM bugs that
zap SPTEs in a very specific window.

Removing the sanity check will allow removing kvm_tdp_mmu_gpa_is_mapped(),
which has no business being exposed to vendor code.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/tdx.c | 14 --------------
 1 file changed, 14 deletions(-)

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 66744f5768c8..a6155f76cc6a 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -3175,20 +3175,6 @@ static int tdx_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 	if (ret < 0)
 		goto out;

-	/*
-	 * The private mem cannot be zapped after kvm_tdp_map_page()
-	 * because all paths are covered by slots_lock and the
-	 * filemap invalidate lock.  Check that they are indeed enough.
-	 */
-	if (IS_ENABLED(CONFIG_KVM_PROVE_MMU)) {
-		scoped_guard(read_lock, &kvm->mmu_lock) {
-			if (KVM_BUG_ON(!kvm_tdp_mmu_gpa_is_mapped(vcpu, gpa), kvm)) {
-				ret = -EIO;
-				goto out;
-			}
-		}
-	}
-
 	ret = 0;
 	err = tdh_mem_page_add(&kvm_tdx->td, gpa, pfn_to_page(pfn),
 			       src_page, &entry, &level_state);
-- 
2.51.0.268.g9569e192d0-goog