Subject: Re: [PATCH v7 3/8] KVM: Make __kvm_follow_pfn not imply FOLL_GET

From: Yu Zhang <[email-masked]>
Date: Wed, 5 Jul 2023 19:56:53 +0800

On Tue, Jul 04, 2023 at 04:50:48PM +0900, David Stevens wrote:
> From: David Stevens <[email-masked]>
> 
> Make it so that __kvm_follow_pfn does not imply FOLL_GET. This allows
> callers to resolve a gfn when the associated pfn has a valid struct page
> that isn't being actively refcounted (e.g. tail pages of non-compound
> higher order pages). For a caller to safely omit FOLL_GET, all usages of
> the returned pfn must be guarded by a mmu notifier.
> 
> This also adds a is_refcounted_page out parameter to kvm_follow_pfn that
> is set when the returned pfn has an associated struct page with a valid
> refcount. Callers that don't pass FOLL_GET should remember this value
> and use it to avoid places like kvm_is_ad_tracked_page that assume a
> non-zero refcount.
> 
> Signed-off-by: David Stevens <[email-masked]>
> ---
>  include/linux/kvm_host.h | 10 ++++++
>  virt/kvm/kvm_main.c      | 67 +++++++++++++++++++++-------------------
>  virt/kvm/pfncache.c      |  2 +-
>  3 files changed, 47 insertions(+), 32 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index ef2763c2b12e..a45308c7d2d9 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1157,6 +1157,9 @@ unsigned long gfn_to_hva_memslot_prot(struct kvm_memory_slot *slot, gfn_t gfn,
>  void kvm_release_page_clean(struct page *page);
>  void kvm_release_page_dirty(struct page *page);
>  
> +void kvm_set_page_accessed(struct page *page);
> +void kvm_set_page_dirty(struct page *page);
> +
>  struct kvm_follow_pfn {
>  	const struct kvm_memory_slot *slot;
>  	gfn_t gfn;
> @@ -1164,10 +1167,17 @@ struct kvm_follow_pfn {
>  	bool atomic;
>  	/* Allow a read fault to create a writeable mapping. */
>  	bool allow_write_mapping;
> +	/*
> +	 * Usage of the returned pfn will be guared by a mmu notifier. Must
> +	 * be true if FOLL_GET is not set.
> +	 */
> +	bool guarded_by_mmu_notifier;

And how? Any place to check the invalidate seq?

B.R.
Yu