Re: [PATCH RFC v4 07/44] KVM: guest_memfd: Only prepare folios for private pages

[Ackerley Tng]

Ackerley Tng <ackerleytng@xxxxxxxxxx> writes:

> All-shared guest_memfd used to be only supported for non-CoCo VMs where
> preparation doesn't apply. INIT_SHARED is about to be supported for
> non-CoCo VMs in a later patch in this series.
>
> In addition, KVM_SET_MEMORY_ATTRIBUTES2 is about to be supported in
> guest_memfd in a later patch in this series.
>
> This means that the kvm fault handler may now call kvm_gmem_get_pfn() on a
> shared folio for a CoCo VM where preparation applies.
>
> Add a check to make sure that preparation is only performed for private
> folios.
>
> Preparation will be undone on freeing (see kvm_gmem_free_folio()) and on
> conversion to shared.
>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> ---
>  virt/kvm/guest_memfd.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index b6ffa8734175d..d414ebfcb4c19 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -900,6 +900,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>  		     int *max_order)
>  {
>  	pgoff_t index = kvm_gmem_get_index(slot, gfn);
> +	struct inode *inode;
>  	struct folio *folio;
>  	int r = 0;
>
> @@ -907,7 +908,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>  	if (!file)
>  		return -EFAULT;
>
> -	filemap_invalidate_lock_shared(file_inode(file)->i_mapping);
> +	inode = file_inode(file);
> +	filemap_invalidate_lock_shared(inode->i_mapping);
>
>  	folio = __kvm_gmem_get_pfn(file, slot, index, pfn, max_order);
>  	if (IS_ERR(folio)) {
> @@ -920,7 +922,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>  		folio_mark_uptodate(folio);
>  	}
>
> -	r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);
> +	if (kvm_gmem_is_private_mem(inode, index))
> +		r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);

Michael, I might have misunderstood you at the last guest_memfd call:
sev_gmem_prepare() doesn't prepare a page for being a shared page,
right? Does this work? That prepare is only called to "make private"?

>
>  	folio_unlock(folio);
>
> @@ -930,7 +933,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>  		folio_put(folio);
>
>  out:
> -	filemap_invalidate_unlock_shared(file_inode(file)->i_mapping);
> +	filemap_invalidate_unlock_shared(inode->i_mapping);
>  	return r;
>  }
>  EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_get_pfn);
>
> --
> 2.53.0.1018.g2bb0e51243-goog