linux-kernel - [PATCH v3 03/25] KVM: TDX: Drop PROVE_MMU=y sanity check on to-be-populated mappings

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Message-ID: <20251017003244.186495-4-seanjc@google.com>
Date: Thu, 16 Oct 2025 17:32:21 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>, 
	Huacai Chen <chenhuacai@...nel.org>, Madhavan Srinivasan <maddy@...ux.ibm.com>, 
	Anup Patel <anup@...infault.org>, Paul Walmsley <pjw@...nel.org>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Christian Borntraeger <borntraeger@...ux.ibm.com>, Janosch Frank <frankja@...ux.ibm.com>, 
	Claudio Imbrenda <imbrenda@...ux.ibm.com>, Sean Christopherson <seanjc@...gle.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, "Kirill A. Shutemov" <kas@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	kvm@...r.kernel.org, loongarch@...ts.linux.dev, linux-mips@...r.kernel.org, 
	linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, x86@...nel.org, linux-coco@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>, 
	Kai Huang <kai.huang@...el.com>, Michael Roth <michael.roth@....com>, 
	Yan Zhao <yan.y.zhao@...el.com>, Vishal Annapurve <vannapurve@...gle.com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Ackerley Tng <ackerleytng@...gle.com>, 
	Binbin Wu <binbin.wu@...ux.intel.com>
Subject: [PATCH v3 03/25] KVM: TDX: Drop PROVE_MMU=y sanity check on
 to-be-populated mappings

Drop TDX's sanity check that a mirror 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 the check
on nr_premapped after TDH.MEM.PAGE.ADD will detect and return an error if
KVM attempted to zap a S-EPT entry (tdx_sept_zap_private_spte() will fail
on TDH.MEM.RANGE.BLOCK due lack of a valid S-EPT entry).  And as evidenced
by the "is mapped?" code being guarded with CONFIG_KVM_PROVE_MMU=y, KVM is
NOT relying on the check for general correctness.

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 (note, this also applies to the check
on nr_premapped).

Removing the sanity check will allow removing kvm_tdp_mmu_gpa_is_mapped(),
which has no business being exposed to vendor code, and more importantly
will pave the way for eliminating the "pre-map" approach entirely in favor
of doing TDH.MEM.PAGE.ADD under mmu_lock.

Reviewed-by: Ira Weiny <ira.weiny@...el.com>
Reviewed-by: Kai Huang <kai.huang@...el.com>
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 326db9b9c567..4c3014befe9f 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -3181,20 +3181,6 @@ 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);
-- 
2.51.0.858.gf9c4a03a3a-goog