From: Borislav Petkov <bp@xxxxxxxxx> 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
