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

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Message-ID: <2a76e282aaa29af234b4aafb2e9485930d3a33b9.camel@intel.com>
Date: Thu, 30 Oct 2025 23:03:13 +0000
From: "Huang, Kai" <kai.huang@...el.com>
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Subject: Re: [PATCH v4 20/28] KVM: TDX: Assert that mmu_lock is held for write
 when removing S-EPT entries

On Thu, 2025-10-30 at 13:09 -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
								^
								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@...gle.com>

Reviewed-by: Kai Huang <kai.huang@...el.com>