linux-kernel - Re: [PATCH 05/22] kvm: mmu: Add functions to handle changed TDP SPTEs

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Message-ID: <af1045cb-2e53-4967-831c-5375ef1c97a2@redhat.com>
Date:   Sat, 26 Sep 2020 02:39:59 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Cannon Matthews <cannonmatthews@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 05/22] kvm: mmu: Add functions to handle changed TDP SPTEs

On 25/09/20 23:22, Ben Gardon wrote:
> +
> +	/*
> +	 * Recursively handle child PTs if the change removed a subtree from
> +	 * the paging structure.
> +	 */
> +	if (was_present && !was_leaf && (pfn_changed || !is_present)) {
> +		pt = spte_to_child_pt(old_spte, level);
> +
> +		for (i = 0; i < PT64_ENT_PER_PAGE; i++) {
> +			old_child_spte = *(pt + i);
> +			*(pt + i) = 0;
> +			handle_changed_spte(kvm, as_id,
> +				gfn + (i * KVM_PAGES_PER_HPAGE(level - 1)),
> +				old_child_spte, 0, level - 1);
> +		}

Is it worth returning a "flush" value to the caller, to avoid multiple
kvm_flush_remote_tlbs_with_address when e.g. zapping a 3rd-level PTE?

Also I prefer if we already include here a "stupid" version of
handle_changed_spte that just calls __handle_changed_spte.  (If my
suggestion is accepted, handle_changed_spte could actually handle the
flushing).

Paolo

> +
> +		kvm_flush_remote_tlbs_with_address(kvm, gfn,
> +						   KVM_PAGES_PER_HPAGE(level));
> +
> +		free_page((unsigned long)pt);
> +	}