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

[Sean Christopherson <seanjc@google.com>]

On Tue, Sep 07, 2021, Brijesh Singh wrote:
> 
> On 9/3/21 2:38 PM, Sean Christopherson wrote:
> > My personal preference is obviously to work towards an abstracted API.  And if
> > we decide to go that route, I think we should be much more aggressive with respect
> > to what is abstracted.   Many of the functions will be rather gross due to the
> > sheer number of params, but I think the end result will be a net positive in terms
> > of readability and separation of concerns.
> > 
> > E.g. get KVM looking like this
> > 
> > static int sev_receive_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
> > {
> > 	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > 	struct kvm_sev_receive_start params;
> > 	int ret;
> > 
> > 	if (!sev_guest(kvm))
> > 		return -ENOTTY;
> > 
> > 	/* Get parameter from the userspace */
> > 	if (copy_from_user(&params, (void __user *)(uintptr_t)argp->data,
> > 			sizeof(struct kvm_sev_receive_start)))
> > 		return -EFAULT;
> > 
> > 	ret = sev_guest_receive_start(argp->sev_fd, &arpg->error, sev->asid,
> > 				      &params.handle, params.policy,
> > 				      params.pdh_uaddr, params.pdh_len,
> > 				      params.session_uaddr, params.session_len);
> > 	if (ret)
> > 		return ret;
> > 
> > 	/* Copy params back to user even on failure, e.g. for error info. */
> > 	if (copy_to_user((void __user *)(uintptr_t)argp->data,
> > 			 &params, sizeof(struct kvm_sev_receive_start)))
> > 		return -EFAULT;
> > 
> >      	sev->handle = params.handle;
> > 	sev->fd = argp->sev_fd;
> > 	return 0;
> > }
> > 
> 
> 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.

We can even do a direct copy to sev_data* with KVM's current UAPI by swapping
fields as necessary, e.g. swap policy<->handle before and after send-start, but
that's all kinds of gross and probably not a net positive.