Re: [PATCH v5 05/13] KVM: Extend the memslot to support fd-based private memory

[Sean Christopherson]

On Thu, Mar 10, 2022, Chao Peng wrote:
> Extend the memslot definition to provide fd-based private memory support
> by adding two new fields (private_fd/private_offset). The memslot then
> can maintain memory for both shared pages and private pages in a single
> memslot. Shared pages are provided by existing userspace_addr(hva) field
> and private pages are provided through the new private_fd/private_offset
> fields.
> 
> Since there is no 'hva' concept anymore for private memory so we cannot
> rely on get_user_pages() to get a pfn, instead we use the newly added
> memfile_notifier to complete the same job.
> 
> This new extension is indicated by a new flag KVM_MEM_PRIVATE.
> 
> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>

Needs a Co-developed-by: for Yu, or a From: if Yu is the sole author.

> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
>  Documentation/virt/kvm/api.rst | 37 +++++++++++++++++++++++++++-------
>  include/linux/kvm_host.h       |  7 +++++++
>  include/uapi/linux/kvm.h       |  8 ++++++++
>  3 files changed, 45 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 3acbf4d263a5..f76ac598606c 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -1307,7 +1307,7 @@ yet and must be cleared on entry.
>  :Capability: KVM_CAP_USER_MEMORY
>  :Architectures: all
>  :Type: vm ioctl
> -:Parameters: struct kvm_userspace_memory_region (in)
> +:Parameters: struct kvm_userspace_memory_region(_ext) (in)
>  :Returns: 0 on success, -1 on error
>  
>  ::
> @@ -1320,9 +1320,17 @@ yet and must be cleared on entry.
>       __u64 userspace_addr; /* start of the userspace allocated memory */
>    };
>  
> +  struct kvm_userspace_memory_region_ext {
> +     struct kvm_userspace_memory_region region;
> +     __u64 private_offset;
> +     __u32 private_fd;
> +     __u32 padding[5];

Uber nit, I'd prefer we pad u32 for private_fd separate from padding the size of
the structure for future expansion.

Regarding future expansion, any reason not to go crazy and pad like 128+ bytes?
It'd be rather embarassing if the next memslot extension needs 3 u64s and we end
up with region_ext2 :-)

> +};
> +
>    /* for kvm_memory_region::flags */
>    #define KVM_MEM_LOG_DIRTY_PAGES    (1UL << 0)
>    #define KVM_MEM_READONLY   (1UL << 1)
> +  #define KVM_MEM_PRIVATE            (1UL << 2)
>  
>  This ioctl allows the user to create, modify or delete a guest physical
>  memory slot.  Bits 0-15 of "slot" specify the slot id and this value

...

> +static inline bool kvm_slot_is_private(const struct kvm_memory_slot *slot)

I 100% think we should usurp the name "private" for these memslots, but as prep
work this series should first rename KVM_PRIVATE_MEM_SLOTS to avoid confusion.
Maybe KVM_INTERNAL_MEM_SLOTS?