linux-kernel - Re: [PATCH 12/14] KVM: x86/mmu: Don't set dirty bits when disabling dirty logging w/ PML

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Message-ID: <9293da48-8bad-0b70-4548-3df7931b6bee@redhat.com>
Date:   Thu, 18 Feb 2021 18:08:09 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Makarand Sonare <makarandsonare@...gle.com>
Subject: Re: [PATCH 12/14] KVM: x86/mmu: Don't set dirty bits when disabling
 dirty logging w/ PML

On 13/02/21 01:50, Sean Christopherson wrote:
> This means that spurious PML entries will be created for memslots with 
> dirty logging disabled if at least one other memslot has dirty logging 
> enabled, but for all known use cases, dirty logging is a global VMM 
> control.

This is not true.  For example QEMU uses dirty logging to track changes 
to the framebuffer.

However, what you're saying below is true: after a MR_CREATE there will 
be no shadow pages, and when they are created with mmu_set_spte they 
will not have the dirty bits set.  So there's really no change here for 
the case of only some memslots having dirty logging enabled.  Queued 12 
and 13 as well then!

Paolo

> Furthermore, spurious PML entries are already possible since 
> dirty bits are set only when a dirty logging is turned off, i.e. 
> memslots that are never dirty logged will have dirty bits cleared.