On Sat, Aug 13, 2022 at 09:04:58AM -0700, Andy Lutomirski wrote:

On Wed, Aug 3, 2022, at 7:02 AM, Dave Hansen wrote:

quoted

On 8/2/22 16:46, Dave Hansen wrote:

quoted

To sum it all up, I'm not happy with the complexity of the page
acceptance code either but I'm not sure that it's bad tradeoff compared
to greater #VE complexity or fragility.

Does anyone think we should go back and really reconsider this?

One other thing I remembered as I re-read my write up on this.

In the "new" mode, guests never get #VE's for unaccepted memory.  They
just exit to the host and can never be reentered.  They must be killed.

In the "old" mode, I _believe_ that the guest always gets a #VE for
non-EPT-present memory.  The #VE is basically the same no matter if the
page is unaccepted or if the host goes out and makes a
previously-accepted page non-present.

One really nasty implication of this "old" mode is that the host can
remove *accepted* pages that are used in the syscall gap.  That means
that the #VE handler would need to be of the paranoid variety which
opens up all kinds of other fun.

 * "Old" - #VE's can happen in the syscall gap
 * "New" - #VE's happen at better-defined times.  Unexpected ones are
   fatal.

There's a third option which I proposed but doesn't yet exist.  The TDX
module _could_ separate the behavior of unaccepted memory #VE's and
host-induced #VEs.  This way, we could use load_unaligned_zeropad() with
impunity and handle it in the #VE handler.  At the same time, the host
would not be allowed to remove accepted memory and cause problems in the
syscall gap.  Kinda the best of both worlds.

But, I'm not sure how valuable that would be now that we have the
(admittedly squirrelly) code to avoid load_unaligned_zeropad() #VE's.

How would that be implemented?  It would need to track which GPAs *were*
accepted across a host-induced unmap/remap cycle. This would involve
preventing the host from ever completely removing a secure EPT table
without the guest’s help, right?

Admittedly this would IMO be better behavior. Is it practical to implement?

I don't think it is better if you look from host POV. It owns resources of
the machine and it has to have a way to pull memory from uncooperative TD
at any point.

It also would require more complicated private->shared conversion as guest
has to notify TDX module about the change, not only host as we do now.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov