Re: [PATCH v4 20/28] KVM: TDX: Assert that mmu_lock is held for write when removing S-EPT entries

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

On 10/31/2025 4:09 AM, Sean Christopherson wrote:
> Unconditionally assert that mmu_lock is held for write when removing S-EPT
> entries, not just when removing S-EPT entries triggers certain conditions,
> e.g. needs to do TDH_MEM_TRACK or kick vCPUs out of the guest.
> Conditionally asserting implies that it's safe to hold mmu_lock for read
> when those paths aren't hit, which is simply not true, as KVM doesn't
> support removing S-EPT entries under read-lock.
>
> Only two paths lead to remove_external_spte(), and both paths asserts that
                                                                 ^
                                                               assert
> mmu_lock is held for write (tdp_mmu_set_spte() via lockdep, and
> handle_removed_pt() via KVM_BUG_ON()).
>
> Deliberately leave lockdep assertions in the "no vCPUs" helpers to document
> that wait_for_sept_zap is guarded by holding mmu_lock for write, and keep
> the conditional assert in tdx_track() as well, but with a comment to help
> explain why holding mmu_lock for write matters (above and beyond why
> tdx_sept_remove_private_spte()'s requirements).
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>

> ---
>   arch/x86/kvm/vmx/tdx.c | 7 +++++++
>   1 file changed, 7 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 63d4609cc3bc..999b519494e9 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -1715,6 +1715,11 @@ static void tdx_track(struct kvm *kvm)
>   	if (unlikely(kvm_tdx->state != TD_STATE_RUNNABLE))
>   		return;
>   
> +	/*
> +	 * The full sequence of TDH.MEM.TRACK and forcing vCPUs out of guest
> +	 * mode must be serialized, as TDH.MEM.TRACK will fail if the previous
> +	 * tracking epoch hasn't completed.
> +	 */
>   	lockdep_assert_held_write(&kvm->mmu_lock);
>   
>   	err = tdh_mem_track(&kvm_tdx->td);
> @@ -1762,6 +1767,8 @@ static void tdx_sept_remove_private_spte(struct kvm *kvm, gfn_t gfn,
>   	gpa_t gpa = gfn_to_gpa(gfn);
>   	u64 err, entry, level_state;
>   
> +	lockdep_assert_held_write(&kvm->mmu_lock);
> +
>   	/*
>   	 * HKID is released after all private pages have been removed, and set
>   	 * before any might be populated. Warn if zapping is attempted when


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