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

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Date:   Mon, 28 Feb 2022 16:11:59 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Hildenbrand <david@...hat.com>,
        kvm <kvm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH v3 07/28] KVM: x86/mmu: Check for !leaf=>leaf, not PFN
 change, in TDP MMU SP removal

On Fri, Feb 25, 2022 at 4:16 PM Sean Christopherson <seanjc@...gle.com> 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@...gle.com>

Reviewed-by: Ben Gardon <bgardon@...gle.com>

> ---
>  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
>