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

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Date:   Tue, 29 Sep 2020 17:07:53 +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 17/22] kvm: mmu: Support dirty logging for the TDP MMU

On 25/09/20 23:22, Ben Gardon wrote:
> +	for_each_tdp_pte_root(iter, root, start, end) {
> +iteration_start:
> +		if (!is_shadow_present_pte(iter.old_spte))
> +			continue;
> +
> +		/*
> +		 * If this entry points to a page of 4K entries, and 4k entries
> +		 * should be skipped, skip the whole page. If the non-leaf
> +		 * entry is at a higher level, move on to the next,
> +		 * (lower level) entry.
> +		 */
> +		if (!is_last_spte(iter.old_spte, iter.level)) {
> +			if (skip_4k && iter.level == PG_LEVEL_2M) {
> +				tdp_iter_next_no_step_down(&iter);
> +				if (iter.valid && iter.gfn >= end)
> +					goto iteration_start;
> +				else
> +					break;

The iteration_start label confuses me mightily. :)  That would be a case
where iter.gfn >= end (so for_each_tdp_pte_root would exit) but you want
to proceed anyway with the gfn that was found by
tdp_iter_next_no_step_down.  Are you sure you didn't mean

	if (iter.valid && iter.gfn < end)
		goto iteration_start;
	else
		break;

because that would make much more sense: basically a "continue" that
skips the tdp_iter_next.  With the min_level change I suggested no
Friday, it would become something like this:

        for_each_tdp_pte_root_level(iter, root, start, end, min_level) {
                if (!is_shadow_present_pte(iter.old_spte) ||
                    !is_last_spte(iter.old_spte, iter.level))
                        continue;

                new_spte = iter.old_spte & ~PT_WRITABLE_MASK;

		*iter.sptep = new_spte;
                handle_change_spte(kvm, as_id, iter.gfn, iter.old_spte,
				   new_spte, iter.level);

                spte_set = true;
                tdp_mmu_iter_cond_resched(kvm, &iter);
        }

which is all nice and understandable.

Also, related to this function, why ignore the return value of
tdp_mmu_iter_cond_resched?  It does makes sense to assign spte_set =
true since, just like in kvm_mmu_slot_largepage_remove_write_access's
instance of slot_handle_large_level, you don't even need to flush on
cond_resched.  However, in order to do that you would have to add some
kind of "bool flush_on_resched" argument to tdp_mmu_iter_cond_resched,
or have two separate functions tdp_mmu_iter_cond_{flush_and_,}resched.

The same is true of clear_dirty_gfn_range and set_dirty_gfn_range.

Paolo