Subject: Re: [PATCH 05/26] vfio: KVM: Pass get/put helpers from KVM to VFIO, don't do circular lookup

From: Jason Gunthorpe <[email-masked]>
Date: Mon, 18 Sep 2023 13:02:58 -0300

On Mon, Sep 18, 2023 at 08:49:57AM -0700, Sean Christopherson wrote:
> On Mon, Sep 18, 2023, Jason Gunthorpe wrote:
> > On Fri, Sep 15, 2023 at 05:30:57PM -0700, Sean Christopherson wrote:
> > > Explicitly pass KVM's get/put helpers to VFIO when attaching a VM to
> > > VFIO instead of having VFIO do a symbol lookup back into KVM.  Having both
> > > KVM and VFIO do symbol lookups increases the overall complexity and places
> > > an unnecessary dependency on KVM (from VFIO) without adding any value.
> > > 
> > > Signed-off-by: Sean Christopherson <[email-masked]>
> > > ---
> > >  drivers/vfio/vfio.h      |  2 ++
> > >  drivers/vfio/vfio_main.c | 74 +++++++++++++++++++---------------------
> > >  include/linux/vfio.h     |  4 ++-
> > >  virt/kvm/vfio.c          |  9 +++--
> > >  4 files changed, 47 insertions(+), 42 deletions(-)
> > 
> > I don't mind this, but Christoph had disliked my prior attempt to do
> > this with function pointers..
> > 
> > The get can be inlined, IIRC, what about putting a pointer to the put
> > inside the kvm struct?
> 
> That wouldn't allow us to achieve our goal, which is to hide the details of
> "struct kvm" from VFIO (and the rest of the kernel).

> What's the objection to handing VFIO a function pointer?

Hmm, looks like it was this thread:

 https://lore.kernel.org/r/[email-masked]

Your rational looks a little better to me.

> > The the normal kvm get/put don't have to exported symbols at all?
> 
> The export of kvm_get_kvm_safe() can go away (I forgot to do that in this series),
> but kvm_get_kvm() will hang around as it's needed by KVM sub-modules (PPC and x86),
> KVMGT (x86), and drivers/s390/crypto/vfio_ap_ops.c (no idea what to call that beast).

My thought would be to keep it as an inline, there should be some way
to do that without breaking your desire to hide the bulk of the kvm
struct content. Like put the refcount as the first element in the
struct and just don't ifdef it away?.

Jason