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

[Vipin Sharma]

On Fri, Sep 25, 2020 at 03:22:20PM -0700, Vipin Sharma wrote:
> On Thu, Sep 24, 2020 at 02:55:18PM -0500, Tom Lendacky 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.
> > 
> > Thanks,
> > Tom
> > 
> > > 
> > > > So, we are talking about 4 different types of resources:
> > > > 1. AMD SEV ASID (implemented in this patch as sev.* files in SEV cgroup)
> > > > 2. AMD SEV-ES ASID (in future, adding files like sev_es.*)
> > > > 3. Intel TDX private KeyID
> > > > 4. Intel TDX shared KeyID
> > > > 
> > > > TDX private KeyID is similar to SEV and SEV-ES ASID. I think coming up
> > > > with the same name which can be used by both platforms will not be easy,
> > > > and extensible with the future enhancements. This will get even more
> > > > difficult if Arm also comes up with something similar but with different
> > > > nuances.
> > > 
> > > Honest question, what's easier for userspace/orchestration layers?  Having an
> > > abstract but common name, or conrete but different names?  My gut reaction is
> > > to provide a common interface, but I can see how that could do more harm than
> > > good, e.g. some amount of hardware capabilitiy discovery is possible with
> > > concrete names.  And I'm guessing there's already a fair amount of vendor
> > > specific knowledge bleeding into userspace for these features...
> 
> I agree with you that the abstract name is better than the concrete
> name, I also feel that we must provide HW extensions. Here is one
> approach:
> 
> Cgroup name: cpu_encryption, encryption_slots, or memcrypt (open to
> suggestions)
> 
> Control files: slots.{max, current, events}
> 
> Contents of the slot.max:
> default max
> 	default: Corresponds to all kinds of encryption capabilities on
> 		 a platform. For AMD, it will be SEV and SEV-ES.  For
> 		 Intel, it will be TDX and MKTME. This can also be used
> 		 by other devices not just CPU.
> 
> 	max: max or any number to denote limit on the cgroup.
> 
> A user who wants the finer control, then they need to know about the
> capabilities a platform provides and use them, e.g. on AMD:
> 
> $ echo "sev-es 1000" > slot.max
> $ cat slots.max
> default max sev-es 1000
> 
> This means that in the cgroup maximum SEV-ES ASIDs which can be used is
> 1000 and SEV ASIDs is max (default, no limit).  Each platform can
> provide their own extensions which can be overwritten by a user,
> otherwise extensions will have the default limit.
> 
> This is kind of similar to the IO and the rdma controller.
> 
> I think it is keeping abstraction for userspace and also providing finer
> control for HW specific features.
> 
> What do you think about the above approach?  
> 
Hi Sean,

Any feedback/concern for the above abstraction approach?

Thanks