Re: [PATCH v3 19/25] KVM: TDX: Assert that mmu_lock is held for write when removing S-EPT entries

[Sean Christopherson <seanjc@google.com>]

On Thu, Oct 23, 2025, Yan Zhao wrote:
> On Thu, Oct 16, 2025 at 05:32:37PM -0700, 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
> > 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()?

Can do.

> 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()).

Does this look/sound right?

From: Sean Christopherson <seanjc@google.com>
Date: Thu, 28 Aug 2025 17:06:17 -0700
Subject: [PATCH] KVM: TDX: Assert that mmu_lock is held for write when
 removing S-EPT entries

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, 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>
---
 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 dca9e2561270..899051c64faa 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

base-commit: 69564844a116861ebea4396894005c8b4e48f870
--

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