linux-kernel - Re: [PATCH v2 1/6] KVM: x86/mmu: Tag disallowed NX huge pages even if they're not tracked

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Date:   Fri, 29 Jul 2022 00:11:00 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v2 1/6] KVM: x86/mmu: Tag disallowed NX huge pages even if
 they're not tracked

On 7/23/22 03:23, Sean Christopherson wrote:
> Tag shadow pages that cannot be replaced with an NX huge page even if
> zapping the page would not allow KVM to create a huge page, e.g. because
> something else prevents creating a huge page.  This will allow a future
> patch to more precisely apply the mitigation by checking if an existing
> shadow page can be replaced by a NX huge page.  Currently, KVM assumes
> that any existing shadow page encountered cannot be replaced by a NX huge
> page (if the mitigation is enabled), which prevents KVM from replacing
> no-longer-necessary shadow pages with huge pages, e.g. after disabling
> dirty logging, zapping from the mmu_notifier due to page migration,
> etc...
> 
> Failure to tag shadow pages appropriately could theoretically lead to
> false negatives, e.g. if a fetch fault requests a small page and thus
> isn't tracked, and a read/write fault later requests a huge page, KVM
> will not reject the huge page as it should.
> 
> To avoid yet another flag, initialize the list_head and use list_empty()
> to determine whether or not a page is on the list of NX huge pages that
> should be recovered.
> 
> Opportunstically rename most of the variables/functions involved to
> provide consistency, e.g. lpage vs huge page and NX huge vs huge NX, and
> clarity, e.g. to make it obvious the flag applies only to the NX huge
> page mitigation, not to any condition that prevents creating a huge page.

Please do this in a separate patch, since this one is already complex 
enough.

>   	 * The following two entries are used to key the shadow page in the
> @@ -100,7 +106,14 @@ struct kvm_mmu_page {
>   		};
>   	};
>   
> -	struct list_head lpage_disallowed_link;
> +	/*
> +	 * Use to track shadow pages that, if zapped, would allow KVM to create

s/Use/Used/

Thanks,

Paolo