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

Michael Kelley (LINUX)
2022-11-30T16:11:33Z

From: Borislav Petkov <bp@alien8.de> Sent: Tuesday, November 29, 2022 9:47 AM
>
> On Tue, Nov 29, 2022 at 03:49:06PM +0000, Michael Kelley (LINUX) wrote:
> > But it turns out that AMD really has two fairly different schemes:
> > the C-bit scheme and the vTOM scheme.
>
> Except it doesn't:
>
> "In the VMSA of an SNP-active guest, the VIRTUAL_TOM field designates
> a 2MB aligned guest physical address called the virtual top of memory.
> When bit 1 (vTOM) of SEV_FEATURES is set in the VMSA of an SNP-active
> VM, the VIRTUAL_TOM..."
>
> So SEV_FEATURES[1] is vTOM and it is part of SNP.
>
> Why do you keep harping on this being something else is beyond me...
>
> I already pointed you to the patch which adds this along with the other
> SEV_FEATURES.
>
> > The details of these two AMD schemes are pretty different. vTOM is
> > *not* just a minor option on the C-bit scheme. It's an either/or -- a
> > guest VM is either doing the C-bit scheme or the vTOM scheme, not some
> > combination. Linux code in coco/core.c could choose to treat C-bit and
> > vTOM as two sub-schemes under CC_VENDOR_AMD, but that makes the code a
> > bit messy because we end up with "if" statements to figure out whether
> > to do things the C-bit way or the vTOM way.
>
> Are you saying that that:
>
> if (cc_vendor == CC_VENDOR_AMD &&
> sev_features & MSR_AMD64_SNP_VTOM_ENABLED)
>
> is messy? Why?
>
> We will have to support vTOM sooner or later.
>
> > Or we could model the two AMD schemes as two different vendors,
> > which is what I'm suggesting. Doing so recognizes that the two schemes
> > are fairly disjoint, and it makes the code cleaner.
>
> How is that any different from the above check?
>
> You *need* some sort of a check to differentiate between the two anyway.
>

Alright. Enough conceptual debate. I'll do a v4 of the patch series with
the AMD C-bit and vTOM schemes folder under CC_VENDOR_AMD and
we can see if there's any further feedback. I should have that v4 out later
today or tomorrow.

Michael