Message-ID: <aPtPhGcE3nqtMDNK@yzhao56-desk.sh.intel.com>
Date: Fri, 24 Oct 2025 18:05:56 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>,
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 Shutemov" <kas@...nel.org>, <linux-arm-kernel@...ts.infradead.org>,
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	Ackerley Tng <ackerleytng@...gle.com>, Binbin Wu <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH v3 19/25] KVM: TDX: Assert that mmu_lock is held for
 write when removing S-EPT entries

On Thu, Oct 23, 2025 at 08:14:04AM -0700, Sean Christopherson wrote:
> 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@...gle.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?
LGTM. Thanks!

> From: Sean Christopherson <seanjc@...gle.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@...gle.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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