Re: [RFC Patch 0/2] KVM: SVM: Cgroup support for SVM SEV ASIDs

[Peter Gonda]

On Thu, Sep 24, 2020 at 1:55 PM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>
> On 9/24/20 2:21 PM, Sean Christopherson wrote:
> > On Tue, Sep 22, 2020 at 02:14:04PM -0700, Vipin Sharma wrote:
> >> On Mon, Sep 21, 2020 at 06:48:38PM -0700, Sean Christopherson wrote:
> >>> On Mon, Sep 21, 2020 at 05:40:22PM -0700, Vipin Sharma wrote:
> >>>> Hello,
> >>>>
> >>>> This patch series adds a new SEV controller for tracking and limiting
> >>>> the usage of SEV ASIDs on the AMD SVM platform.
> >>>>
> >>>> SEV ASIDs are used in creating encrypted VM and lightweight sandboxes
> >>>> but this resource is in very limited quantity on a host.
> >>>>
> >>>> This limited quantity creates issues like SEV ASID starvation and
> >>>> unoptimized scheduling in the cloud infrastructure.
> >>>>
> >>>> SEV controller provides SEV ASID tracking and resource control
> >>>> mechanisms.
> >>>
> >>> This should be genericized to not be SEV specific.  TDX has a similar
> >>> scarcity issue in the form of key IDs, which IIUC are analogous to SEV ASIDs
> >>> (gave myself a quick crash course on SEV ASIDs).  Functionally, I doubt it
> >>> would change anything, I think it'd just be a bunch of renaming.  The hardest
> >>> part would probably be figuring out a name :-).
> >>>
> >>> Another idea would be to go even more generic and implement a KVM cgroup
> >>> that accounts the number of VMs of a particular type, e.g. legacy, SEV,
> >>> SEV-ES?, and TDX.  That has potential future problems though as it falls
> >>> apart if hardware every supports 1:MANY VMs:KEYS, or if there is a need to
> >>> account keys outside of KVM, e.g. if MKTME for non-KVM cases ever sees the
> >>> light of day.
> >>
> >> I read about the TDX and its use of the KeyID for encrypting VMs. TDX
> >> has two kinds of KeyIDs private and shared.
> >
> > To clarify, "shared" KeyIDs are simply legacy MKTME KeyIDs.  This is relevant
> > because those KeyIDs can be used without TDX or KVM in the picture.
> >
> >> On AMD platform there are two types of ASIDs for encryption.
> >> 1. SEV ASID - Normal runtime guest memory encryption.
> >> 2. SEV-ES ASID - Extends SEV ASID by adding register state encryption with
> >>               integrity.
> >>
> >> Both types of ASIDs have their own maximum value which is provisioned in
> >> the firmware
> >
> > Ugh, I missed that detail in the SEV-ES RFC.  Does SNP add another ASID type,
> > or does it reuse SEV-ES ASIDs?  If it does add another type, is that trend
> > expected to continue, i.e. will SEV end up with SEV, SEV-ES, SEV-ES-SNP,
> > SEV-ES-SNP-X, SEV-ES-SNP-X-Y, etc...?
>
> SEV-SNP and SEV-ES share the same ASID range.

Where is this documented? From the SEV-SNP FW ABI Spec 0.8 "The
firmware checks that ASID is an encryption capable ASID. If not, the
firmware returns INVALID_ASID." that doesn't seem clear that an SEV-ES
ASID is required. Should this document be more clear?