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

[Chao Peng]

On Mon, Mar 28, 2022 at 09:56:33PM +0000, Sean Christopherson wrote:
> 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@xxxxxxxxxxxxxxx>
> > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > ---
> >  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.

Will try that.

> 
> 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__

Is this #ifndef? As I think anonymous struct is only for kernel?

Thanks,
Chao

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