Re: [PATCH 26/27] KVM: x86/mmu: Add page-track API to query if a gfn is valid

[Yan Zhao]

On Fri, Dec 23, 2022 at 12:57:38AM +0000, Sean Christopherson wrote:
> Add a page-track API to query if a gfn is "valid", i.e. is backed by a
> memslot and is visible to the guest.  This is one more step toward
> removing KVM internal details from the page-track APIs.
> 
> Add a FIXME to call out that intel_gvt_is_valid_gfn() is broken with
> respect to 2MiB (or larger) guest entries, e.g. if the starting gfn is
> valid but a 2MiB page starting at the gfn covers "invalid" memory due
> to running beyond the memslot.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_page_track.h |  1 +
>  arch/x86/kvm/mmu/page_track.c         | 13 +++++++++++++
>  drivers/gpu/drm/i915/gvt/gtt.c        | 11 ++---------
>  3 files changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
> index 66a0d7c34311..99e1d6eeb0fb 100644
> --- a/arch/x86/include/asm/kvm_page_track.h
> +++ b/arch/x86/include/asm/kvm_page_track.h
> @@ -52,6 +52,7 @@ int kvm_page_track_register_notifier(struct kvm *kvm,
>  void kvm_page_track_unregister_notifier(struct kvm *kvm,
>  					struct kvm_page_track_notifier_node *n);
>  
> +bool kvm_page_track_is_valid_gfn(struct kvm *kvm, gfn_t gfn);
>  int kvm_write_track_add_gfn(struct kvm *kvm, gfn_t gfn);
>  int kvm_write_track_remove_gfn(struct kvm *kvm, gfn_t gfn);
>  #endif /* CONFIG_KVM_EXTERNAL_WRITE_TRACKING */
> diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
> index 1af431a41f71..9da071a514b3 100644
> --- a/arch/x86/kvm/mmu/page_track.c
> +++ b/arch/x86/kvm/mmu/page_track.c
> @@ -264,6 +264,19 @@ enum pg_level kvm_page_track_max_mapping_level(struct kvm *kvm, gfn_t gfn,
>  }
>  EXPORT_SYMBOL_GPL(kvm_page_track_max_mapping_level);
>  
> +bool kvm_page_track_is_valid_gfn(struct kvm *kvm, gfn_t gfn)
> +{
> +	bool ret;
> +	int idx;
> +
> +	idx = srcu_read_lock(&kvm->srcu);
> +	ret = kvm_is_visible_gfn(kvm, gfn);
> +	srcu_read_unlock(&kvm->srcu, idx);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(kvm_page_track_is_valid_gfn);
This implementation is only to check whether a GFN is within a visible
kvm memslot. So, why this helper function is named kvm_page_track_xxx()?
Don't think it's anything related to page track, and not all of its callers
in KVMGT are for page tracking.

Thanks
Yan

> +
>  /*
>   * add guest page to the tracking pool so that corresponding access on that
>   * page will be intercepted.
> diff --git a/drivers/gpu/drm/i915/gvt/gtt.c b/drivers/gpu/drm/i915/gvt/gtt.c
> index 59ba6639e622..43c4fc23205d 100644
> --- a/drivers/gpu/drm/i915/gvt/gtt.c
> +++ b/drivers/gpu/drm/i915/gvt/gtt.c
> @@ -51,18 +51,11 @@ static int preallocated_oos_pages = 8192;
>  
>  static bool intel_gvt_is_valid_gfn(struct intel_vgpu *vgpu, unsigned long gfn)
>  {
> -	struct kvm *kvm = vgpu->vfio_device.kvm;
> -	int idx;
> -	bool ret;
> -
>  	if (!vgpu->attached)
>  		return false;
>  
> -	idx = srcu_read_lock(&kvm->srcu);
> -	ret = kvm_is_visible_gfn(kvm, gfn);
> -	srcu_read_unlock(&kvm->srcu, idx);
> -
> -	return ret;
> +	/* FIXME: This doesn't properly handle guest entries larger than 4K. */
> +	return kvm_page_track_is_valid_gfn(vgpu->vfio_device.kvm, gfn);
>  }
>  
>  /*
> -- 
> 2.39.0.314.g84b9a713c41-goog
>