linux-kernel - Re: [PATCH 1/2] KVM: x86/mmu: Don't skip non-leaf SPTEs when zapping all SPTEs

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Message-ID: <01b22936-49b0-638e-baf8-269ba93facd8@redhat.com>
Date:   Thu, 12 Aug 2021 18:18:24 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 1/2] KVM: x86/mmu: Don't skip non-leaf SPTEs when zapping
 all SPTEs

On 12/08/21 07:07, Sean Christopherson wrote:
> @@ -739,8 +749,16 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>   			  gfn_t start, gfn_t end, bool can_yield, bool flush,
>   			  bool shared)
>   {
> +	bool zap_all = (end == ZAP_ALL_END);
>   	struct tdp_iter iter;
>   
> +	/*
> +	 * Bound the walk at host.MAXPHYADDR, guest accesses beyond that will
> +	 * hit a #PF(RSVD) and never get to an EPT Violation/Misconfig / #NPF,
> +	 * and so KVM will never install a SPTE for such addresses.
> +	 */
> +	end = min(end, 1ULL << (shadow_phys_bits - PAGE_SHIFT));

Then zap_all need not have any magic value.  You can use 0/-1ull, it's 
readable enough.  ZAP_ALL_END is also unnecessary here if you do:

	gfn_t max_gfn_host = 1ULL << (shadow_phys_bits - PAGE_SHIFT);
	bool zap_all = (start == 0 && end >= max_gfn_host);

	end = min(end, max_gfn_host);

And as a small commit message nit, I would say "don't leak" instead of 
"don't skip", since that's really the effect.

Paolo


>   	kvm_lockdep_assert_mmu_lock_held(kvm, shared);
>   
>   	rcu_read_lock();
> @@ -759,9 +777,10 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>   		/*
>   		 * If this is a non-last-level SPTE that covers a larger range
>   		 * than should be zapped, continue, and zap the mappings at a
> -		 * lower level.
> +		 * lower level, except when zapping all SPTEs.
>   		 */
> -		if ((iter.gfn < start ||
> +		if (!zap_all &&
> +		    (iter.gfn < start ||
>   		     iter.gfn + KVM_PAGES_PER_HPAGE(iter.level) > end) &&