Re: [PATCH v2 1/9] KVM: x86: Add AMD SEV specific Hypercall3

[Sean Christopherson <seanjc@google.com>]

On Sun, Dec 06, 2020, Paolo Bonzini wrote:
> On 03/12/20 01:34, Sean Christopherson wrote:
> > On Tue, Dec 01, 2020, Ashish Kalra wrote:
> > > From: Brijesh Singh <brijesh.singh@amd.com>
> > > 
> > > KVM hypercall framework relies on alternative framework to patch the
> > > VMCALL -> VMMCALL on AMD platform. If a hypercall is made before
> > > apply_alternative() is called then it defaults to VMCALL. The approach
> > > works fine on non SEV guest. A VMCALL would causes #UD, and hypervisor
> > > will be able to decode the instruction and do the right things. But
> > > when SEV is active, guest memory is encrypted with guest key and
> > > hypervisor will not be able to decode the instruction bytes.
> > > 
> > > Add SEV specific hypercall3, it unconditionally uses VMMCALL. The hypercall
> > > will be used by the SEV guest to notify encrypted pages to the hypervisor.
> > 
> > What if we invert KVM_HYPERCALL and X86_FEATURE_VMMCALL to default to VMMCALL
> > and opt into VMCALL?  It's a synthetic feature flag either way, and I don't
> > think there are any existing KVM hypercalls that happen before alternatives are
> > patched, i.e. it'll be a nop for sane kernel builds.
> > 
> > I'm also skeptical that a KVM specific hypercall is the right approach for the
> > encryption behavior, but I'll take that up in the patches later in the series.
> 
> Do you think that it's the guest that should "donate" memory for the bitmap
> instead?

No.  Two things I'd like to explore:

  1. Making the hypercall to announce/request private vs. shared common across
     hypervisors (KVM, Hyper-V, VMware, etc...) and technologies (SEV-* and TDX).
     I'm concerned that we'll end up with multiple hypercalls that do more or
     less the same thing, e.g. KVM+SEV, Hyper-V+SEV, TDX, etc...  Maybe it's a
     pipe dream, but I'd like to at least explore options before shoving in KVM-
     only hypercalls.

  2. Tracking shared memory via a list of ranges instead of a using bitmap to
     track all of guest memory.  For most use cases, the vast majority of guest
     memory will be private, most ranges will be 2mb+, and conversions between
     private and shared will be uncommon events, i.e. the overhead to walk and
     split/merge list entries is hopefully not a big concern.  I suspect a list
     would consume far less memory, hopefully without impacting performance.