Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

[Sean Christopherson]

On Thu, Sep 08, 2022, Kirill A. Shutemov wrote:
> On Wed, Aug 31, 2022 at 05:24:39PM +0300, Kirill A . Shutemov wrote:
> > On Sat, Aug 20, 2022 at 10:15:32PM -0700, Hugh Dickins wrote:
> > > > I will try next week to rework it as shim to top of shmem. Does it work
> > > > for you?
> > > 
> > > Yes, please do, thanks.  It's a compromise between us: the initial TDX
> > > case has no justification to use shmem at all, but doing it that way
> > > will help you with some of the infrastructure, and will probably be
> > > easiest for KVM to extend to other more relaxed fd cases later.
> > 
> > Okay, below is my take on the shim approach.
> > 
> > I don't hate how it turned out. It is easier to understand without
> > callback exchange thing.
> > 
> > The only caveat is I had to introduce external lock to protect against
> > race between lookup and truncate.

As before, I think this lock is unnecessary.  Or at least it's unnessary to hold
the lock across get/put.  The ->invalidate() call will ensure that the pfn is
never actually used if get() races with truncation.

Switching topics, what actually prevents mmapp() on the shim?  I tried to follow,
but I don't know these areas well enough.