Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

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On Wed, Jun 19, 2024, Fuad Tabba wrote:
> Hi Jason,
> 
> On Wed, Jun 19, 2024 at 12:51 PM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> >
> > On Wed, Jun 19, 2024 at 10:11:35AM +0100, Fuad Tabba wrote:
> >
> > > To be honest, personally (speaking only for myself, not necessarily
> > > for Elliot and not for anyone else in the pKVM team), I still would
> > > prefer to use guest_memfd(). I think that having one solution for
> > > confidential computing that rules them all would be best. But we do
> > > need to be able to share memory in place, have a plan for supporting
> > > huge pages in the near future, and migration in the not-too-distant
> > > future.
> >
> > I think using a FD to control this special lifetime stuff is
> > dramatically better than trying to force the MM to do it with struct
> > page hacks.
> >
> > If you can't agree with the guest_memfd people on how to get there
> > then maybe you need a guest_memfd2 for this slightly different special
> > stuff instead of intruding on the core mm so much. (though that would
> > be sad)
> >
> > We really need to be thinking more about containing these special
> > things and not just sprinkling them everywhere.
> 
> I agree that we need to agree :) This discussion has been going on
> since before LPC last year, and the consensus from the guest_memfd()
> folks (if I understood it correctly) is that guest_memfd() is what it
> is: designed for a specific type of confidential computing, in the
> style of TDX and CCA perhaps, and that it cannot (or will not) perform
> the role of being a general solution for all confidential computing.

That isn't remotely accurate.  I have stated multiple times that I want guest_memfd
to be a vehicle for all VM types, i.e. not just CoCo VMs, and most definitely not
just TDX/SNP/CCA VMs.

What I am staunchly against is piling features onto guest_memfd that will cause
it to eventually become virtually indistinguishable from any other file-based
backing store.  I.e. while I want to make guest_memfd usable for all VM *types*,
making guest_memfd the preferred backing store for all *VMs* and use cases is
very much a non-goal.


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