Re: [PATCH v5 3/9] KVM: guest_memfd: Allow host to map guest_memfd() pages

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Mon, Mar 03, 2025 at 05:10:07PM +0000, Fuad Tabba wrote:
> Add support for mmap() and fault() for guest_memfd backed memory
> in the host for VMs that support in-place conversion between
> shared and private. To that end, this patch adds the ability to
> check whether the VM type supports in-place conversion, and only
> allows mapping its memory if that's the case.
> 
> Also add the KVM capability KVM_CAP_GMEM_SHARED_MEM, which
> indicates that the VM supports shared memory in guest_memfd, or
> that the host can create VMs that support shared memory.
> Supporting shared memory implies that memory can be mapped when
> shared with the host.
> 
> This is controlled by the KVM_GMEM_SHARED_MEM configuration
> option.
> 
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  include/linux/kvm_host.h |  11 ++++
>  include/uapi/linux/kvm.h |   1 +
>  virt/kvm/guest_memfd.c   | 105 +++++++++++++++++++++++++++++++++++++++
>  virt/kvm/kvm_main.c      |   4 ++
>  4 files changed, 121 insertions(+)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 7788e3625f6d..2d025b8ee20e 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -728,6 +728,17 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm)
>  }
>  #endif
>  
> +/*
> + * Arch code must define kvm_arch_gmem_supports_shared_mem if support for
> + * private memory is enabled and it supports in-place shared/private conversion.
> + */
> +#if !defined(kvm_arch_gmem_supports_shared_mem) && !IS_ENABLED(CONFIG_KVM_PRIVATE_MEM)

Hm. Do we expect any caller for !CONFIG_KVM_PRIVATE_MEM?

> +static inline bool kvm_arch_gmem_supports_shared_mem(struct kvm *kvm)
> +{
> +	return false;
> +}
> +#endif
> +
>  #ifndef kvm_arch_has_readonly_mem
>  static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)
>  {

... 

> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index b2aa6bf24d3a..4291956b51ae 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -312,7 +312,112 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
>  	return gfn - slot->base_gfn + slot->gmem.pgoff;
>  }
>  
> +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
> +static bool kvm_gmem_offset_is_shared(struct file *file, pgoff_t index)
> +{
> +	struct kvm_gmem *gmem = file->private_data;
> +
> +	/* For now, VMs that support shared memory share all their memory. */
> +	return kvm_arch_gmem_supports_shared_mem(gmem->kvm);
> +}
> +
> +static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
> +{
> +	struct inode *inode = file_inode(vmf->vma->vm_file);
> +	struct folio *folio;
> +	vm_fault_t ret = VM_FAULT_LOCKED;
> +
> +	filemap_invalidate_lock_shared(inode->i_mapping);
> +
> +	folio = kvm_gmem_get_folio(inode, vmf->pgoff);
> +	if (IS_ERR(folio)) {
> +		switch (PTR_ERR(folio)) {
> +		case -EAGAIN:
> +			ret = VM_FAULT_RETRY;
> +			break;
> +		case -ENOMEM:
> +			ret = VM_FAULT_OOM;
> +			break;
> +		default:
> +			ret = VM_FAULT_SIGBUS;
> +			break;
> +		}
> +		goto out_filemap;
> +	}
> +
> +	if (folio_test_hwpoison(folio)) {
> +		ret = VM_FAULT_HWPOISON;
> +		goto out_folio;
> +	}
> +
> +	/* Must be called with folio lock held, i.e., after kvm_gmem_get_folio() */

If this is a requirement, it would be cleaner to rename the function and
pass down the folio and check the lock state inside.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov