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

[Kirill A. Shutemov]

On Tue, Sep 13, 2022 at 09:44:27AM +0000, Sean Christopherson wrote:
> 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.

The updated version you replying to does not use the lock to protect
against truncation anymore. The lock protect notifier list.

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

It has no f_op->mmap, so mmap() will fail with -ENODEV. See do_mmap().
(I did not read the switch statement correctly at first. Note there are
two 'fallthrough' there.)

-- 
  Kiryl Shutsemau / Kirill A. Shutemov