RE: [PATCH v2 0/8] x86/coco: Mark CoCo VM pages not present when changing encrypted state

[Michael Kelley]

From: kirill.shutemov@xxxxxxxxxxxxxxx <kirill.shutemov@xxxxxxxxxxxxxxx> Sent: Friday, November 24, 2023 2:06 AM
> 
> On Tue, Nov 21, 2023 at 01:20:08PM -0800, mhkelley58@xxxxxxxxx wrote:
> > From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> >
> > In a CoCo VM when a page transitions from encrypted to decrypted, or vice
> > versa, attributes in the PTE must be updated *and* the hypervisor must
> > be notified of the change.
> 
> Strictly speaking it is not true for TDX. Conversion to shared can be
> implicit: set shared bit and touch the page will do the conversion. MapGPA
> is optional.

Interesting.  Given that, is there a reason to use the explicit
hypervisor callbacks in for private->shared transitions in 
__set_mem_enc_pgtable()?   It probably doesn't have direct relevance
to this patch series, but I'm just trying to understand the tradeoffs of
the implicit vs. explicit approach.  And am I correct that
shared->private transitions must use the explicit approach?

> 
> > Because there are two separate steps, there's
> > a window where the settings are inconsistent.  Normally the code that
> > initiates the transition (via set_memory_decrypted() or
> > set_memory_encrypted()) ensures that the memory is not being accessed
> > during a transition, so the window of inconsistency is not a problem.
> > However, the load_unaligned_zeropad() function can read arbitrary memory
> > pages at arbitrary times, which could read a transitioning page during
> > the window.  In such a case, CoCo VM specific exceptions are taken
> > (depending on the CoCo architecture in use).  Current code in those
> > exception handlers recovers and does "fixup" on the result returned by
> > load_unaligned_zeropad().  Unfortunately, this exception handling can't
> > work in paravisor scenarios (TDX Paritioning and SEV-SNP in vTOM mode)
> > if the exceptions are routed to the paravisor.  The paravisor can't
> > do load_unaligned_zeropad() fixup, so the exceptions would need to
> > be forwarded from the paravisor to the Linux guest, but there are
> > no architectural specs for how to do that.
> 
> Hm. Can't we inject #PF (or #GP) into L2 if #VE/#VC handler in L1 sees
> cross-page access to shared memory while no fixup entry for the page in
> L1. It would give L2 chance to handle the situation in a transparent way.
> 
> Maybe I miss something, I donno.

I'm recounting what the Hyper-V paravisor folks say without knowing all the
details. :-(   But it seems like any kind of forwarding scheme needs to be a
well-defined contract that would work for both TDX and SEV-SNP.   The
paravisor in L1 might or might not be Linux-based, so the contract must be OS
independent.  And the L2 guest might or might not be Linux, so there's
potential for some other kind of error to be confused with a Linux
load_unaligned_zeropad() reference.

Maybe all that could be sorted out, but I come back to believing that it's
better now and in the long run to just avoid all this complexity by decoupling
private-shared page transitions and Linux load_unaligned_zeropad().
Unfortunately that decoupling hasn't been as simple as I first envisioned
because of SEV-SNP PVALIDATE needing a virtual address.  But doing the
decoupling only in the paravisor case still seems like the simpler approach.

Michael

> 
> --
>   Kiryl Shutsemau / Kirill A. Shutemov