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    SubjectRe: [PATCH v2 20/27] KVM: x86/mmu: Use page-track notifiers iff there are external users
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    On Wed, Mar 15, 2023, Sean Christopherson wrote:
    > On Wed, Mar 15, 2023, Yan Zhao wrote:
    > > On Fri, Mar 10, 2023 at 04:22:51PM -0800, Sean Christopherson wrote:
    > > > Disable the page-track notifier code at compile time if there are no
    > > > external users, i.e. if CONFIG_KVM_EXTERNAL_WRITE_TRACKING=n. KVM itself
    > > > now hooks emulated writes directly instead of relying on the page-track
    > > > mechanism.
    > > >
    > > > Signed-off-by: Sean Christopherson <seanjc@google.com>
    > > > ---
    > > > arch/x86/include/asm/kvm_host.h | 2 ++
    > > > arch/x86/include/asm/kvm_page_track.h | 2 ++
    > > > arch/x86/kvm/mmu/page_track.c | 9 ++++-----
    > > > arch/x86/kvm/mmu/page_track.h | 29 +++++++++++++++++++++++----
    > > > 4 files changed, 33 insertions(+), 9 deletions(-)
    > > >
    > > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
    > > > index 1a4225237564..a3423711e403 100644
    > > > --- a/arch/x86/include/asm/kvm_host.h
    > > > +++ b/arch/x86/include/asm/kvm_host.h
    > > > @@ -1265,7 +1265,9 @@ struct kvm_arch {
    > > > * create an NX huge page (without hanging the guest).
    > > > */
    > > > struct list_head possible_nx_huge_pages;
    > > > +#ifdef CONFIG_KVM_EXTERNAL_WRITE_TRACKING
    > > > struct kvm_page_track_notifier_head track_notifier_head;
    > > > +#endif
    > > > /*
    > > > * Protects marking pages unsync during page faults, as TDP MMU page
    > > > * faults only take mmu_lock for read. For simplicity, the unsync
    > > > diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
    > > > index deece45936a5..53c2adb25a07 100644
    > > > --- a/arch/x86/include/asm/kvm_page_track.h
    > > > +++ b/arch/x86/include/asm/kvm_page_track.h
    > > The "#ifdef CONFIG_KVM_EXTERNAL_WRITE_TRACKING" can be moved to the
    > > front of this file?
    > > All the structures are only exposed for external users now.
    >
    > Huh. I've no idea why I didn't do that. IIRC, the entire reason past me wrapped
    > track_notifier_head in an #ifdef was to allow this change in kvm_page_track.h.
    >
    > I'll do this in the next version unless I discover an edge case I'm overlooking.

    Ah, deja vu. I tried this first time around, and got yelled at by the kernel test
    robot. Unsuprisingly, my second attempt yielded the same result :-)

    HDRTEST drivers/gpu/drm/i915/gvt/gvt.h
    In file included from <command-line>:
    gpu/drivers/gpu/drm/i915/gvt/gvt.h:236:45: error: field ‘track_node’ has incomplete type
    236 | struct kvm_page_track_notifier_node track_node;
    | ^~~~~~~~~~

    The problem is direct header inclusion. Nothing in the kernel includes gvt.h
    when CONFIG_DRM_I915_GVT=n, but the header include guard tests include headers
    directly on the command line. I think I'll define a "stub" specifically to play
    nice with this sort of testing. Guarding the guts of gvt.h with CONFIG_DRM_I915_GVT
    would just propagate the problem, and guarding the node definition in "struct
    intel_vgpu" would be confusing since the guard would be dead code for all intents
    and purposes.

    The obvious alternative would be to leave kvm_page_track_notifier_node outside of
    the #ifdef, but I really want to bury kvm_page_track_notifier_head for KVM's sake,
    and having "head" buried but not "node" would also be weird and confusing.

    diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
    index 33f087437209..3d040741044b 100644
    --- a/arch/x86/include/asm/kvm_page_track.h
    +++ b/arch/x86/include/asm/kvm_page_track.h
    @@ -51,6 +51,12 @@ void kvm_page_track_unregister_notifier(struct kvm *kvm,

    int kvm_write_track_add_gfn(struct kvm *kvm, gfn_t gfn);
    int kvm_write_track_remove_gfn(struct kvm *kvm, gfn_t gfn);
    +#else
    +/*
    + * Allow defining a node in a structure even if page tracking is disabled, e.g.
    + * to play nice with testing headers via direct inclusion from the command line.
    + */
    +struct kvm_page_track_notifier_node {};
    #endif /* CONFIG_KVM_EXTERNAL_WRITE_TRACKING */

    #endif
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