linux-kernel - Re: [PATCH] KVM: x86/MMU: Zap non-leaf SPTEs when disabling dirty logging

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Message-ID: <a1fbab86-ece9-82e3-64fe-0a19a125513b@redhat.com>
Date:   Thu, 26 May 2022 17:52:44 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Ben Gardon <bgardon@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Dunn <daviddunn@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Junaid Shahid <junaids@...gle.com>
Subject: Re: [PATCH] KVM: x86/MMU: Zap non-leaf SPTEs when disabling dirty
 logging

On 5/26/22 16:30, Peter Xu wrote:
> On Thu, May 26, 2022 at 02:01:43PM +0200, Paolo Bonzini wrote:
>> On 5/26/22 01:09, Ben Gardon wrote:
>>> +		WARN_ON(max_mapping_level < iter.level);
>>> +
>>> +		/*
>>> +		 * If this page is already mapped at the highest
>>> +		 * viable level, there's nothing more to do.
>>> +		 */
>>> +		if (max_mapping_level == iter.level)
>>> +			continue;
>>> +
>>> +		/*
>>> +		 * The page can be remapped at a higher level, so step
>>> +		 * up to zap the parent SPTE.
>>> +		 */
>>> +		while (max_mapping_level > iter.level)
>>> +			tdp_iter_step_up(&iter);
>>> +
>>>    		/* Note, a successful atomic zap also does a remote TLB flush. */
>>> -		if (tdp_mmu_zap_spte_atomic(kvm, &iter))
>>> -			goto retry;
>>> +		tdp_mmu_zap_spte_atomic(kvm, &iter);
>>> +
>>
>> Can you make this a sparate function (for example
>> tdp_mmu_zap_collapsible_spte_atomic)?  Otherwise looks great!
> 
> There could be a tiny downside of using a helper in that it'll hide the
> step-up of the iterator, which might not be as obvious as keeping it in the
> loop?

That's true, my reasoning is that zapping at a higher level can only be 
done by first moving the iterator up.  Maybe 
tdp_mmu_zap_at_level_atomic() is a better Though, I can very well apply 
this patch as is.

Paolo