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

[Sean Christopherson <seanjc@google.com>]

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>
> 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;

Peeking ahead, the partial switch to the _ext variant is rather gross.  I would
prefer that KVM use an entirely different, but binary compatible, struct internally.
And once the kernel supports C11[*], I'm pretty sure we can make the "region" in
_ext an anonymous struct, and make KVM's internal struct a #define of _ext.  That
should minimize the churn (no need to get the embedded "region" field), reduce
line lengths, and avoid confusion due to some flows taking the _ext but others
dealing with only the "base" struct.

Maybe kvm_user_memory_region or kvm_user_mem_region?  Though it's tempting to be
evil and usurp the old kvm_memory_region :-)

E.g. pre-C11 do

struct kvm_userspace_memory_region_ext {
	struct kvm_userspace_memory_region region;
	__u64 private_offset;
	__u32 private_fd;
	__u32 padding[5];
};

#ifdef __KERNEL__
struct kvm_user_mem_region {
	__u32 slot;
	__u32 flags;
	__u64 guest_phys_addr;
	__u64 memory_size; /* bytes */
	__u64 userspace_addr; /* start of the userspace allocated memory */
	__u64 private_offset;
	__u32 private_fd;
	__u32 padding[5];
};
#endif

and then post-C11 do

struct kvm_userspace_memory_region_ext {
#ifdef __KERNEL__
	struct kvm_userspace_memory_region region;
#else
	struct kvm_userspace_memory_region;
#endif
	__u64 private_offset;
	__u32 private_fd;
	__u32 padding[5];
};

#ifdef __KERNEL__
#define kvm_user_mem_region kvm_userspace_memory_region_ext
#endif

[*] https://lore.kernel.org/all/20220301145233.3689119-1-arnd@kernel.org

> +	__u64 private_offset;
> +	__u32 private_fd;
> +	__u32 padding[5];
> +};