linux-kernel - Re: [RFC PATCH v3 04/19] KVM: x86: mmu: allow to enable write tracking externally

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Message-ID: <YoZyWOh4NPA0uN5J@google.com>
Date:   Thu, 19 May 2022 16:37:44 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ingo Molnar <mingo@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        intel-gfx@...ts.freedesktop.org, Daniel Vetter <daniel@...ll.ch>,
        Borislav Petkov <bp@...en8.de>, Joerg Roedel <joro@...tes.org>,
        linux-kernel@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Brijesh Singh <brijesh.singh@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        intel-gvt-dev@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [RFC PATCH v3 04/19] KVM: x86: mmu: allow to enable write
 tracking externally

On Wed, Apr 27, 2022, Maxim Levitsky wrote:
> @@ -5753,6 +5752,10 @@ int kvm_mmu_init_vm(struct kvm *kvm)
>  	node->track_write = kvm_mmu_pte_write;
>  	node->track_flush_slot = kvm_mmu_invalidate_zap_pages_in_memslot;
>  	kvm_page_track_register_notifier(kvm, node);

Can you add a patch to move this call to kvm_page_track_register_notifier() into
mmu_enable_write_tracking(), and simultaneously add a WARN in the register path
that page tracking is enabled?

Oh, actually, a better idea. Add an inner __kvm_page_track_register_notifier()
that is not exported and thus used only by KVM, invoke mmu_enable_write_tracking()
from the exported kvm_page_track_register_notifier(), and then do the above.
That will require modifying KVMGT and KVM in a single patch, but that's ok.

That will avoid any possibility of an external user failing to enabling tracking
before registering its notifier, and also avoids bikeshedding over what to do with
the one-line wrapper to enable tracking.