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

From: "Michael Kelley (LINUX)" <mikelley@microsoft.com>
To: Borislav Petkov <bp@alien8.de>
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Subject: RE: [Patch v3 07/14] x86/hyperv: Change vTOM handling to use standard coco mechanisms
Date: Wed, 30 Nov 2022 16:11:18 +0000	[thread overview]
Message-ID: <BYAPR21MB168891CB2831C9BAF829EADBD7159@BYAPR21MB1688.namprd21.prod.outlook.com> (raw)
In-Reply-To: <Y4ZFmktxPlEjyoeR@zn.tnic>

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