On 8/29/2025 8:06 AM, Sean Christopherson wrote:
> Drop TDX's sanity check that an S-EPT mapping isn't zapped between creating
^
should be M-EPT?
> 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).
I think this also needs to be updated.
As Yan mentioned in https://lore.kernel.org/lkml/aK6+TQ0r1j5j2PCx@yzhao56-desk.sh.intel.com/
TDH.MEM.PAGE.ADD would succeed without error, but error is still can be detected
via the value of nr_premapped in the end.
>
> 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.
>
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> Reviewed-by: Kai Huang <kai.huang@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.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 6784aaaced87..71da245d160f 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);