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

[Sean Christopherson]

On Tue, Sep 13, 2022, Kirill A. Shutemov wrote:
> 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.

Gah, grabbed the patch when applying.

> > 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.)

Ah, validate_mmap_request().  Thought not implementing ->mmap() was the key, but
couldn't find the actual check.

Thanks much!