RE: [Patch v3 07/14] x86/hyperv: Change vTOM handling to use standard coco mechanisms

Michael Kelley (LINUX)
2022-11-23T00:59:09Z

From: Borislav Petkov <bp@alien8.de> Sent: Tuesday, November 22, 2022 2:18 PM
>
> On Tue, Nov 22, 2022 at 06:22:46PM +0000, Michael Kelley (LINUX) wrote:
> > I think the core problem here is the naming and meaning of
> > CC_VENDOR_HYPERV. The name was created originally when the
> > Hyper-V vTOM handling code was a lot of special cases. With the
> > changes in this patch series that make the vTOM functionality more
> > mainstream, the name would be better as CC_VENDOR_AMD_VTOM.
>
> No, if CC_VENDOR_HYPERV means different things depending on what kind of
> guests you're doing, then you should not use a CC_VENDOR at all.

Agreed. My proposal is to drop CC_VENDOR_HYPERV entirely.
Replace it with CC_VENDOR_AMD_VTOM (or something like that) that
is set *only* by Linux guests that are running on AMD SEV-SNP processors
and using the vTOM scheme instead of the AMD C-bit scheme.

>
> > vTOM is part of the AMD SEV-SNP spec, and it's a different way of
> > doing the encryption from the "C-bit" based approach. As much as
> > possible, I'm trying to not make it be Hyper-V specific, though
> > currently we have N=1 for hypervisors that offer the vTOM option, so
> > it's a little hard to generalize.
>
> Actually, it is very simple to generalize: vTOM and the paravisor and
> VMPL are all part of the effort to support unenlightened, unmodified
> guests with SNP.
>
> So, if KVM wants to run Windows NT 4.0 guests as SNP guests, then it
> probably would need the same contraptions.

Yes, agreed. My point about generalization is that Hyper-V is the only
actual implementation today. Edge cases, like whether the IO-APIC is
accessed as encrypted or as decrypted don't have a pattern yet. But
that's not a blocker. Such cases can be resolved or special-cased later
when/if N > 1.

>
> > With the thinking oriented that way, a Linux guest on Hyper-V using
> > TDX will run with CC_VENDOR_INTEL. A Linux guest on Hyper-V that
> > is fully enlightened to use the "C-bit" will run with CC_VENDOR_AMD.
>
> Right.

Good. We're in agreement. :-)

>
> > Dexuan Cui just posted a patch set for initial TDX support on Hyper-V,
> > and I think that runs with CC_VENDOR_INTEL (Dexuan -- correct me if
> > I'm wrong about that -- I haven't reviewed your patches yet).

I confirmed with Dexuan that his new patch set for TDX guests on Hyper-V
has the guest running with CC_VENDOR_INTEL, which is what we want.

> > Tianyu Lan
> > has a patch set out for Hyper-V guests using the "C-bit". That patch set
> > still uses CC_VENDOR_HYPERV. Tianyu and I need to work through
> > whether his patch set can run with CC_VENDOR_AMD like everyone
> > else using the "C-bit" approach.

I haven't followed up with Tianyu yet.

>
> So I'm not sure the vendor is the right approach here. I guess we need
> to specify the *type* of guest being supported.

Yes, calling it the "vendor" turns out to not quite be right because in
the AMD case, the technology/architecture/scheme/"type" (or
whatever you want to call it) is not 1:1 with the vendor. Intel has just
one (TDX) while AMD has two (C-bit and vTOM). "vendor" is just a label,
but we should get the label right to avoid future confusion. The key point
is that we'll have three top-level types:

* TDX
* AMD with C-bit (and this has some sub-types)
* AMD with vTOM

The CC_ATTR_* values are then derived from the "type".

>
> > Yes, the polarity of the AMD vTOM bit matches the polarity of the
> > TDX GPA.SHARED bit, and is the opposite polarity of the AMD "C-bit".
> > I'll add a comment to that effect.
> >
> > Anyway, that's where I think this should go. Does it make sense?
> > Other thoughts?
>
> I think all that polarity doesn't matter as long as we abstract it away
> with, "mark encrypted" and "mark decrypted".

Agreed.

Michael