Re: [PATCH v3 kvm/queue 04/16] KVM: Extend the memslot to support fd-based private memory

[Sean Christopherson]

On Thu, Dec 23, 2021, Chao Peng wrote:
> Extend the memslot definition to provide fd-based private memory support
> by adding two new fields(fd/ofs). The memslot then can maintain memory
> for both shared and private pages in a single memslot. Shared pages are
> provided in the existing way by using userspace_addr(hva) field and
> get_user_pages() while private pages are provided through the new
> fields(fd/ofs). Since there is no 'hva' concept anymore for private
> memory we cannot call get_user_pages() to get a pfn, instead we rely on
> the newly introduced MEMFD_OPS callbacks to do 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>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
>  include/linux/kvm_host.h | 10 ++++++++++
>  include/uapi/linux/kvm.h | 12 ++++++++++++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index f8ed799e8674..2cd35560c44b 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -460,8 +460,18 @@ struct kvm_memory_slot {
>       u32 flags;
>       short id;
>       u16 as_id;
> +     u32 fd;

There should be no need to store the fd in the memslot, the fd should be 
unneeded
outside of __kvm_set_memory_region(), e.g.

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 1caebded52c4..4e43262887a3 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2029,10 +2029,10 @@ int __kvm_set_memory_region(struct kvm *kvm,
        new->npages = npages;
        new->flags = mem->flags;
        new->userspace_addr = mem->userspace_addr;
-       new->fd = mem->fd;
-       new->file = NULL;
-       new->ofs = mem->ofs;
-
+       if (mem->flags & KVM_MEM_PRIVATE) {
+               new->private_file = fget(mem->private_fd);
+               new->private_offset = mem->private_offset;
+       }
        r = kvm_set_memslot(kvm, old, new, change);
        if (r)
                kfree(new);

> +     struct file *file;

Please use more descriptive names, shaving characters is not at all priority.

> +     u64 ofs;

I believe this should be loff_t.

        struct file *private_file;
        struct loff_t private_offset;

>  };
>  
> +static inline bool kvm_slot_is_private(const struct kvm_memory_slot *slot)
> +{
> +     if (slot && (slot->flags & KVM_MEM_PRIVATE))
> +             return true;
> +     return false;

        return slot && (slot->flags & KVM_MEM_PRIVATE);

> +}
> +
>  static inline bool kvm_slot_dirty_track_enabled(const struct kvm_memory_slot 
> *slot)
>  {
>       return slot->flags & KVM_MEM_LOG_DIRTY_PAGES;
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 1daa45268de2..41434322fa23 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -103,6 +103,17 @@ struct kvm_userspace_memory_region {
>       __u64 userspace_addr; /* start of the userspace allocated memory */
>  };
>  
> +struct kvm_userspace_memory_region_ext {
> +     __u32 slot;
> +     __u32 flags;
> +     __u64 guest_phys_addr;
> +     __u64 memory_size; /* bytes */
> +     __u64 userspace_addr; /* hva */

Would it make sense to embed "struct kvm_userspace_memory_region"?

> +     __u64 ofs; /* offset into fd */
> +     __u32 fd;

Again, use descriptive names, then comments like "offset into fd" are 
unnecessary.

        __u64 private_offset;
        __u32 private_fd;

> +     __u32 padding[5];
> +};
> +
>  /*
>   * The bit 0 ~ bit 15 of kvm_memory_region::flags are visible for userspace,
>   * other bits are reserved for kvm internal use which are defined in
> @@ -110,6 +121,7 @@ struct kvm_userspace_memory_region {
>   */
>  #define KVM_MEM_LOG_DIRTY_PAGES      (1UL << 0)
>  #define KVM_MEM_READONLY     (1UL << 1)
> +#define KVM_MEM_PRIVATE              (1UL << 2)
>  
>  /* for KVM_IRQ_LINE */
>  struct kvm_irq_level {
> -- 
> 2.17.1
>