linux-kernel - Re: [PATCH 05/10] KVM: x86: MMU: Use for_each_rmap_spte macro instead of pte_list

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <564944A3.5080005@lab.ntt.co.jp>
Date:	Mon, 16 Nov 2015 11:51:15 +0900
From:	Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	pbonzini@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/10] KVM: x86: MMU: Use for_each_rmap_spte macro instead
 of pte_list_walk()

On 2015/11/14 18:20, Marcelo Tosatti wrote:

> The actual issue is this: a higher level page that had, under its children,
> no out of sync pages, now, due to your addition, a child that is unsync:
>
> initial state:
> 	level1
>
> final state:
>
> 	level1 -x-> level2 -x-> level3
>
> Where -x-> are the links created by this pagefault fixing round.
>
> If _any_ page under you is unsync (not necessarily the ones this
> pagefault is accessing), you have to mark parents unsync.

I understand this, but I don't think my patch will break this.

What kvm_mmu_mark_parents_unsync() does is:

   for each p_i in sp->parent_ptes rmap chain
     mark_unsync(p_i);

Then, mark_unsync() finds the parent sp including that p_i to
set ->unsync_child_bitmap and increment ->unsync_children if
necessary.  It may also call kvm_mmu_mark_parents_unsync()
recursively.

I understand we need to tell the parents "you have an unsync
child/descendant" until this information reaches the top level
by that recursive calls.

But since these recursive calls cannot come back to the starting sp,
the child->parent graph has no loop, each mark_unsync(p_i) will not
be affected by other parents in that sp->parent_ptes rmap chain,
from which we started the recursive calls.

As the following code shows, my patch does mark_unsync(parent_pte)
separately, and then mmu_page_add_parent_pte(vcpu, sp, parent_pte):

> -		} else if (sp->unsync)
> +			if (parent_pte)
> +				mark_unsync(parent_pte);
> +		} else if (sp->unsync) {
>  			kvm_mmu_mark_parents_unsync(sp);
> +			if (parent_pte)
> +				mark_unsync(parent_pte);
> +		}
> +		mmu_page_add_parent_pte(vcpu, sp, parent_pte);

So, as you worried, during each mark_unsync(p_i) is processed,
this parent_pte does not exist in that sp->parent_ptes rmap chain.

But as I explained above, this does not change anything about what
each mark_unsync(p_i) call does, so keeps the original behaviour.

By the way, I think "kvm_mmu_mark_parents_unsync" and "mark_unsync"
do not tell what they actually do well. When I first saw the names,
I thought they would just set the parents' sp->unsync.

To reflect the following meaning better, it should be
propagate_unsync(_to_parents) or something:

   Tell the parents "you have an unsync child/descendant"
   until this unsync information reaches the top level

Thanks,
   Takuya

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/