Re: [PATCH v2 3/4] KVM: SVM: move sev_bind_asid to psp

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 09, 2021, Brijesh Singh wrote:
> 
> On 9/7/21 6:37 PM, Sean Christopherson wrote:
> > On Tue, Sep 07, 2021, Brijesh Singh wrote:
> > > I have no strong preference for either of the abstraction approaches. The
> > > sheer number of argument can also make some folks wonder whether such
> > > abstraction makes it easy to read. e.g send-start may need up to 11.
> > 
> > Yeah, that's brutal, but IMO having a few ugly functions is an acceptable cost if
> > it means the rest of the API is cleaner.  E.g. KVM is not the right place to
> > implement sev_deactivate_lock, as any coincident DEACTIVATE will be problematic.
> > The current code "works" because KVM is the only in-tree user, but even that's a
> > bit of a grey area because sev_guest_deactivate() is exported.
> > 
> > If large param lists are problematic, one idea would be to reuse the sev_data_*
> > structs for the API.  I still don't like the idea of exposing those structs
> > outside of the PSP driver, and the potential user vs. kernel pointer confusion
> > is more than a bit ugly.  On the other hand it's not exactly secret info,
> > e.g. KVM's UAPI structs are already excrutiatingly close to sev_data_* structs.
> > 
> > For future ioctls(), KVM could even define UAPI structs that are bit-for-bit
> > compatible with the hardware structs.  That would allow KVM to copy userspace's
> > data directly into a "struct sev_data_*" and simply require the handle and any
> > other KVM-defined params to be zero.  KVM could then hand the whole struct over
> > to the PSP driver for processing.
> 
> Most of the address field in the "struct sev_data_*" are physical
> addressess. The userspace will not be able to populate those fields.

Yeah, that's my biggest hesitation to using struct sev_data_* in the API, it's
both confusing and gross.  But it's also why I think these helpers belong in the
PSP driver, KVM should not need to know the "on-the-wire" format for communicating
with the PSP.

> PSP or KVM may still need to assist filling the final hardware structure.
> Some of fields in hardware structure must be zero, so we need to add checks
> for it.

> I can try posting RFC post SNP series and we can see how it all looks.

I'm a bit torn.  I completely understand the desire to get SNP support merged, but
at the same time KVM has accrued a fair bit of technical debt for SEV and SEV-ES,
and the lack of tests is also a concern.  I don't exactly love the idea of kicking
those cans further down the road.

Paolo, any thoughts?