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

[Binbin Wu <binbin.wu@linux.intel.com>]

On 10/17/2025 8:32 AM, Sean Christopherson wrote:
> 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@intel.com>
> Reviewed-by: Kai Huang <kai.huang@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.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);