Re: [PATCH v8 2/8] KVM: Extend the memslot to support fd-based private memory

[Jarkko Sakkinen]

On Thu, Oct 06, 2022 at 05:58:03PM +0300, Jarkko Sakkinen wrote:
> On Thu, Sep 15, 2022 at 10:29:07PM +0800, Chao Peng wrote:
> > This new extension, indicated by the new flag KVM_MEM_PRIVATE, adds two
> > additional KVM memslot fields private_fd/private_offset to allow
> > userspace to specify that guest private memory provided from the
> > private_fd and guest_phys_addr mapped at the private_offset of the
> > private_fd, spanning a range of memory_size.
> > 
> > The extended memslot can still have the userspace_addr(hva). When use, a
> > single memslot can maintain both private memory through private
> > fd(private_fd/private_offset) and shared memory through
> > hva(userspace_addr). Whether the private or shared part is visible to
> > guest is maintained by other KVM code.
> 
> What is anyway the appeal of private_offset field, instead of having just
> 1:1 association between regions and files, i.e. one memfd per region?
> 
> If this was the case, then an extended struct would not be needed in the
> first place. A simple union inside the existing struct would do:
> 
>         union {
>                 __u64 userspace_addr,
>                 __u64 private_fd,
>         };

Also, why is this mechanism just for fd's with MFD_INACCESSIBLE flag? I'd
consider instead having KVM_MEM_FD flag. For generic KVM (if memfd does not
have MFD_INACCESSIBLE set), KVM could just use the memory as it is using
mapped memory. This would simplify user space code, as you can the use the
same thing for both cases.

BR, Jarkko