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

From: Borislav Petkov <[email-masked]>
Date: Tue, 29 Nov 2022 18:47:06 +0100

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.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette