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

[Tom Lendacky]

On 10/1/20 1:08 PM, Peter Gonda wrote:
> 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?

I let the owner of the spec know and it will be updated.

Thanks,
Tom

>