Re: [PATCH v7 6/6] x86/efi: Safely enable unaccepted memory in UEFI

[Dave Hansen <dave.hansen@intel.com>]

On 4/5/23 06:44, Ard Biesheuvel wrote:
> Given that the intent here is to retain compatibility with
> unenlightened workloads (i.e., which do not upgrade their kernels), I
> think it is perfectly reasonable to drop this from mainline at some
> point.

OK, so there are three firmware types that matter:

1. Today's SEV-SNP deployed firmware.
2. Near future SEV-SNP firmware that exposes the new ExitBootServices()
   protocol that allows guests that speak the protocol to boot faster
   by participating in the unaccepted memory dance.
3. Far future firmware that doesn't have the ExitBootServices() protocol

There are also three kernel types:
1. Old kernels with zero unaccepted memory support: no
   ExitBootServices() protocol support and no hypercalls to accept pages
2. Kernels that can accept pages and twiddle the ExitBootServices() flag
3. Future kernels that can accept pages, but have had ExitBootServices()
   support removed.

That leads to nine possible mix-and-match firmware/kernel combos.  I'm
personally assuming that folks are going to *try* to run with all of
these combos and will send us kernel folks bug reports if they see
regressions.  Let's just enumerate all of them and their implications
before we go consult our crystal balls about what folks will actually do
in the future.

So, here we go:

              |                   Kernel                   |
              |                                            |
              | Unenlightened | Enlightened | Dropped UEFI |
Firmware      |     ~5.19??   |    ~6.4??   | protocol     |
              |---------------+-------------+--------------|
Deployed      |   Slow boot   |  Slow boot  |  Slow boot   |
Near future   |   Slow boot   |  Fast boot  |  Slow boot   |
Far future    |   Crashes??   |  Fast Boot  |  Fast boot   |

I hope I got that all right.

The thing that worries me is the "Near future firmware" where someone
runs a ~6.4 kernel and has a fast boot experience.  They upgrade to a
newer, "dropped protocol" kernel and their boot gets slower.

I'm also a little fuzzy about what an ancient enlightened kernel would
do on a "far future" firmware that requires unaccepted memory support.
I _think_ those kernels would hit some unaccepted memory, and
#VC/#VE/#whatever and die.  Is that right, or is there some fallback there?