Re: [PATCH v3 07/28] KVM: x86/mmu: Check for !leaf=>leaf, not PFN change, in TDP MMU SP removal

[Mingwei Zhang]

On Sat, Feb 26, 2022, Sean Christopherson wrote:
> Look for a !leaf=>leaf conversion instead of a PFN change when checking
> if a SPTE change removed a TDP MMU shadow page.  Convert the PFN check
> into a WARN, as KVM should never change the PFN of a shadow page (except
> when its being zapped or replaced).
> 
> From a purely theoretical perspective, it's not illegal to replace a SP
> with a hugepage pointing at the same PFN.  In practice, it's impossible
> as that would require mapping guest memory overtop a kernel-allocated SP.
> Either way, the check is odd.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 189f21e71c36..848448b65703 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -505,9 +505,12 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
>  
>  	/*
>  	 * Recursively handle child PTs if the change removed a subtree from
> -	 * the paging structure.
> +	 * the paging structure.  Note the WARN on the PFN changing without the
> +	 * SPTE being converted to a hugepage (leaf) or being zapped.  Shadow
> +	 * pages are kernel allocations and should never be migrated.
>  	 */
> -	if (was_present && !was_leaf && (pfn_changed || !is_present))
> +	if (was_present && !was_leaf &&
> +	    (is_leaf || !is_present || WARN_ON_ONCE(pfn_changed)))
>  		handle_removed_pt(kvm, spte_to_child_pt(old_spte, level), shared);
>  }
>  
> -- 
> 2.35.1.574.g5d30c73bfb-goog
>