Re: [PATCH v3 19/25] KVM: TDX: Assert that mmu_lock is held for write when removing S-EPT entries
From: Yan Zhao <hidden>
Date: 2025-10-23 07:39:05
Also in:
kvm, kvm-riscv, kvmarm, linux-arm-kernel, linux-coco, linux-mips, linux-riscv, lkml, loongarch
On Thu, Oct 16, 2025 at 05:32:37PM -0700, Sean Christopherson wrote:
quoted hunk ↗ jump to hunk
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 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. Signed-off-by: Sean Christopherson <seanjc@google.com> --- arch/x86/kvm/vmx/tdx.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c index e517ad3d5f4f..f6782b0ffa98 100644 --- a/arch/x86/kvm/vmx/tdx.c +++ b/arch/x86/kvm/vmx/tdx.c@@ -1711,8 +1711,6 @@ static void tdx_track(struct kvm *kvm) if (unlikely(kvm_tdx->state != TD_STATE_RUNNABLE)) return; - lockdep_assert_held_write(&kvm->mmu_lock);
Could we also deliberately leave lockdep assertion for tdx_track()? This is because if we allow removing S-EPT entries while holding mmu_lock for read in future, tdx_track() needs to be protected by a separate spinlock to ensure serialization of tdh_mem_track() and vCPUs kick-off (kicking off vCPUs must follow each tdh_mem_track() to unblock the next tdh_mem_track()).
quoted hunk ↗ jump to hunk
err = tdh_mem_track(&kvm_tdx->td); if (unlikely(tdx_operand_busy(err))) { /* After no vCPUs enter, the second retry is expected to succeed */@@ -1758,6 +1756,8 @@ 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-- 2.51.0.858.gf9c4a03a3a-goog