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

[Chao Peng]

On Mon, Mar 28, 2022 at 09:27:32PM +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@linux.intel.com>
> 
> Needs a Co-developed-by: for Yu, or a From: if Yu is the sole author.

Yes a Co-developed-by for Yu is needed, for all the patches throught the series.

> 
> > 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 :-)

OK, so maybe:
        __u64 private_offset;
        __u32 private_fd;
        __u32 pad1;
        __u32 pad2[28];
> 
> > +};
> > +
> >    /* 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?

Oh, I didn't realized 'PRIVATE' is already taken.  KVM_INTERNAL_MEM_SLOTS
sounds good.

Thanks,
Chao