Re: [PATCH v10 12/16] KVM: x86: Introduce new KVM_FEATURE_SEV_LIVE_MIGRATION feature & Custom MSR.

[Steve Rutherford <srutherford@google.com>]

On Wed, Feb 10, 2021 at 2:01 PM Steve Rutherford <srutherford@google.com> wrote:
>
> Hi Ashish,
>
> On Wed, Feb 10, 2021 at 12:37 PM Ashish Kalra <ashish.kalra@amd.com> wrote:
> >
> > Hello Steve,
> >
> > We can remove the implicit enabling of this live migration feature
> > from svm_vcpu_after_set_cpuid() callback invoked afer KVM_SET_CPUID2
> > ioctl, and let this feature flag be controlled by the userspace
> > VMM/qemu.
> >
> > Userspace can set this feature flag explicitly by calling the
> > KVM_SET_CPUID2 ioctl and enable this feature whenever it is ready to
> > do so.
> >
> > I have tested this as part of Qemu code :
> >
> > int kvm_arch_init_vcpu(CPUState *cs)
> > {
> > ...
> > ...
> >         c->function = KVM_CPUID_FEATURES | kvm_base;
> >         c->eax = env->features[FEAT_KVM];
> >         c->eax |= (1 << KVM_FEATURE_SEV_LIVE_MIGRATION);
> > ...
> > ...
> >
> >     r = kvm_vcpu_ioctl(cs, KVM_SET_CPUID2, &cpuid_data);
> > ...
> >
> > Let me know if this addresses your concerns.
> Removing implicit enablement is one part of the equation.
> The other two are:
> 1) Host userspace being able to ask the kernel if it supports SEV Live Migration
> 2) Host userspace being able to disable access to the MSR/hypercall
>
> Feature flagging for paravirt features is pretty complicated, since
> you need all three parties to negotiate (host userspace/host
> kernel/guest), and every single one has veto power. In the end, the
> feature should only be available to the guest if every single party
> says yes.
>
> For an example of how to handle 1), the new feature flag could be
> checked when asking the kernel which cpuid bits it supports by adding
> it to the list of features that the kernel mentions in
> KVM_GET_SUPPORTED_CPUID.
>
> For example (in KVM's arch/x86/kvm/cpuid.c):
> case KVM_CPUID_FEATURES:
> ==========
> entry->eax = (1 << KVM_FEATURE_CLOCKSOURCE) |
>     (1 << KVM_FEATURE_NOP_IO_DELAY) |
> ...
>     (1 << KVM_FEATURE_PV_SCHED_YIELD) |
> +  (1 << KVM_FEATURE_ASYNC_PF_INT) |
> -   (1 << KVM_FEATURE_ASYNC_PF_INT);
> +  (1 << KVM_FEATURE_SEV_LIVE_MIGRATION);
> ==========
>
> Without this, userspace has to infer if the kernel it is on supports that flag.
>
> For an example of how to handle 2), in the new msr handler, KVM should
> throw a GP `if (!guest_pv_has(vcpu, KVM_FEATURE_SEV_LIVE_MIGRATION))`
> (it can do this by returning th. The issue here is "what if the guest
Correction: (it can do this by returning 1).