Re: [PATCH v6 1/5] x86/kvm: Add AMD SEV specific Hypercall3

[Sean Christopherson <seanjc@google.com>]

On Wed, Sep 15, 2021, Steve Rutherford wrote:
> Looking at these threads, this patch either:
> 1) Needs review/approval from a maintainer that is interested or
> 2) Should flip back to using alternative (as suggested by Sean). In
> particular: `ALTERNATIVE("vmmcall", "vmcall",
> ALT_NOT(X86_FEATURE_VMMCALL))`. My understanding is that the advantage
> of this is that (after calling apply alternatives) you get exactly the
> same behavior as before. But before apply alternatives, you get the
> desired flipped behavior. The previous patch changed the behavior
> after apply alternatives in a very slight manner (if feature flags
> were not set, you'd get a different instruction).
> 
> I personally don't have strong feelings on this decision, but this
> decision does need to be made for this patch series to move forward.
> 
> I'd also be curious to hear Sean's opinion on this since he was vocal
> about this previously.

Pulling in Ashish's last email from the previous thread, which I failed to respond
to.

https://lore.kernel.org/all/20210820133223.GA28059@ashkalra_ubuntu_server/T/#u

On Fri, Aug 20, 2021, Ashish Kalra wrote:
> On Thu, Aug 19, 2021 at 11:15:26PM +0000, Sean Christopherson wrote:
> > On Thu, Aug 19, 2021, Kalra, Ashish wrote:
> > >
> > > > On Aug 20, 2021, at 3:38 AM, Kalra, Ashish <Ashish.Kalra@amd.com> wrote:
> > > > I think it makes more sense to stick to the original approach/patch, i.e.,
> > > > introducing a new private hypercall interface like kvm_sev_hypercall3() and
> > > > let early paravirtualized kernel code invoke this private hypercall
> > > > interface wherever required.
> >
> > I don't like the idea of duplicating code just because the problem is tricky to
> > solve.  Right now it's just one function, but it could balloon to multiple in
> > the future.  Plus there's always the possibility of a new, pre-alternatives
> > kvm_hypercall() being added in generic code, at which point using an SEV-specific
> > variant gets even uglier.

...

> Now, apply_alternatives() is called much later when setup_arch() calls
> check_bugs(), so we do need some kind of an early, pre-alternatives
> hypercall interface.
>
> Other cases of pre-alternatives hypercalls include marking per-cpu GHCB
> pages as decrypted on SEV-ES and per-cpu apf_reason, steal_time and
> kvm_apic_eoi as decrypted for SEV generally.
>
> Actually using this kvm_sev_hypercall3() function may be abstracted
> quite nicely. All these early hypercalls are made through
> early_set_memory_XX() interfaces, which in turn invoke pv_ops.
>
> Now, pv_ops can have this SEV/TDX specific abstractions.
>
> Currently, pv_ops.mmu.notify_page_enc_status_changed() callback is setup
> to kvm_sev_hypercall3() in case of SEV.
>
> Similarly, in case of TDX, pv_ops.mmu.notify_page_enc_status_changed() can
> be setup to a TDX specific callback.
>
> Therefore, this early_set_memory_XX() -> pv_ops.mmu.notify_page_enc_status_changed()
> is a generic interface and can easily have SEV, TDX and any other future platform
> specific abstractions added to it.

Unless there's some fundamental technical hurdle I'm overlooking, if pv_ops can
be configured early enough to handle this, then so can alternatives.  Adding
notify_page_enc_status_changed() may be necessary in the future, e.g. for TDX
or SNP, but IMO that is orthogonal to adding a generic, 100% redundant helper.

I appreciate that simply swapping the default from VMCALL->VMMCALL is a bit dirty
since it gives special meaning to the default value, but if that's the argument
against reusing kvm_hypercall3() then we should solve the early alternatives
problem, not fudge around it.