linux-kernel - Re: [RFC PATCH 01/12] KVM: TDX: Drop PROVE_MMU=y sanity check on to-be-populated mappings

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Message-ID: <68afa4b8bc180_315529475@iweiny-mobl.notmuch>
Date: Wed, 27 Aug 2025 19:37:12 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
	<pbonzini@...hat.com>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Michael Roth
	<michael.roth@....com>, Yan Zhao <yan.y.zhao@...el.com>, Ira Weiny
	<ira.weiny@...el.com>, Vishal Annapurve <vannapurve@...gle.com>, "Rick
 Edgecombe" <rick.p.edgecombe@...el.com>
Subject: Re: [RFC PATCH 01/12] KVM: TDX: Drop PROVE_MMU=y sanity check on
 to-be-populated mappings

Sean Christopherson wrote:
> Drop TDX's sanity check that an S-EPT mapping isn't zapped between creating
> said mapping and doing TDH.MEM.PAGE.ADD, as the check is simultaneously
> superfluous and incomplete.  Per commit 2608f1057601 ("KVM: x86/tdp_mmu:
> Add a helper function to walk down the TDP MMU"), the justification for
> introducing kvm_tdp_mmu_gpa_is_mapped() was to check that the target gfn
> was pre-populated, with a link that points to this snippet:
> 
>  : > One small question:
>  : >
>  : > What if the memory region passed to KVM_TDX_INIT_MEM_REGION hasn't been pre-
>  : > populated?  If we want to make KVM_TDX_INIT_MEM_REGION work with these regions,
>  : > then we still need to do the real map.  Or we can make KVM_TDX_INIT_MEM_REGION
>  : > return error when it finds the region hasn't been pre-populated?
>  :
>  : Return an error.  I don't love the idea of bleeding so many TDX details into
>  : userspace, but I'm pretty sure that ship sailed a long, long time ago.
> 
> But that justification makes little sense for the final code, as simply
> doing TDH.MEM.PAGE.ADD without a paranoid sanity check will return an error
> if the S-EPT mapping is invalid (as evidenced by the code being guarded
> with CONFIG_KVM_PROVE_MMU=y).
> 
> The sanity check is also incomplete in the sense that mmu_lock is dropped
> between the check and TDH.MEM.PAGE.ADD, i.e. will only detect KVM bugs that
> zap SPTEs in a very specific window.
> 
> Removing the sanity check will allow removing kvm_tdp_mmu_gpa_is_mapped(),
> which has no business being exposed to vendor code.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Ira Weiny <ira.weiny@...el.com>

[snip]