linux-kernel - Re: [PATCH 17/22] kvm: mmu: Support dirty logging for the TDP MMU

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Date:   Wed, 30 Sep 2020 11:04:42 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.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 17/22] kvm: mmu: Support dirty logging for the TDP MMU

On Fri, Sep 25, 2020 at 02:22:57PM -0700, Ben Gardon wrote:
> +/*
> + * Remove write access from all the SPTEs mapping GFNs in the memslot. If
> + * skip_4k is set, SPTEs that map 4k pages, will not be write-protected.
> + * Returns true if an SPTE has been changed and the TLBs need to be flushed.
> + */
> +bool kvm_tdp_mmu_wrprot_slot(struct kvm *kvm, struct kvm_memory_slot *slot,
> +			     bool skip_4k)
> +{
> +	struct kvm_mmu_page *root;
> +	int root_as_id;
> +	bool spte_set = false;
> +
> +	for_each_tdp_mmu_root(kvm, root) {
> +		root_as_id = kvm_mmu_page_as_id(root);
> +		if (root_as_id != slot->as_id)
> +			continue;

This pattern pops up quite a few times, probably worth adding

#define for_each_tdp_mmu_root_using_memslot(...)	\
	for_each_tdp_mmu_root(...)			\
		if (kvm_mmu_page_as_id(root) != slot->as_id) {
		} else

> +
> +		/*
> +		 * Take a reference on the root so that it cannot be freed if
> +		 * this thread releases the MMU lock and yields in this loop.
> +		 */
> +		get_tdp_mmu_root(kvm, root);
> +
> +		spte_set = wrprot_gfn_range(kvm, root, slot->base_gfn,
> +				slot->base_gfn + slot->npages, skip_4k) ||
> +			   spte_set;
> +
> +		put_tdp_mmu_root(kvm, root);
> +	}
> +
> +	return spte_set;
> +}
> +
> +/*
> + * Clear the dirty status of all the SPTEs mapping GFNs in the memslot. If
> + * AD bits are enabled, this will involve clearing the dirty bit on each SPTE.
> + * If AD bits are not enabled, this will require clearing the writable bit on
> + * each SPTE. Returns true if an SPTE has been changed and the TLBs need to
> + * be flushed.
> + */
> +static bool clear_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
> +			   gfn_t start, gfn_t end)
> +{
> +	struct tdp_iter iter;
> +	u64 new_spte;
> +	bool spte_set = false;
> +	int as_id = kvm_mmu_page_as_id(root);
> +
> +	for_each_tdp_pte_root(iter, root, start, end) {
> +		if (!is_shadow_present_pte(iter.old_spte) ||
> +		    !is_last_spte(iter.old_spte, iter.level))
> +			continue;

Same thing here, extra wrappers would probably be helpful.  At least add one
for the present case, e.g.

  #define for_each_present_tdp_pte_using_root()

and maybe even

  #define for_each_leaf_tdp_pte_using_root()

since the "!present || !last" pops up 4 or 5 times.

> +
> +		if (spte_ad_need_write_protect(iter.old_spte)) {
> +			if (is_writable_pte(iter.old_spte))
> +				new_spte = iter.old_spte & ~PT_WRITABLE_MASK;
> +			else
> +				continue;