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

[Sean Christopherson]

On Tue, Sep 21, 2021, Borislav Petkov wrote:
> On Tue, Sep 21, 2021 at 01:50:09PM +0000, Sean Christopherson wrote:
> > apply_alternatives() is a generic helper that can work on any struct alt_instr
> > array, e.g. KVM_HYPERCALL can put its alternative into a different section that's
> > patched as soon as the VMM is identified.
> 
> Where exactly in the boot process you wanna move it?

init_hypervisor_platform(), after x86_init.hyper.init_platform() so that the
PV support can set the desired feature flags.  Since kvm_hypercall*() is only
used by KVM guests, set_cpu_cap(c, X86_FEATURE_VMMCALL) can be moved out of
early_init_amd/hygon() and into kvm_init_platform().

> As Ashish says, you need the boot_cpu_data bits properly set before it
> runs.

Another option would be to refactor apply_alternatives() to allow the caller to
provide a different feature check mechanism than boot_cpu_has(), which I think
would let us drop X86_FEATURE_VMMCALL, X86_FEATURE_VMCALL, and X86_FEATURE_VMW_VMMCALL
from cpufeatures.  That might get more than a bit gross though.

But like I said, if others think I'm over-engineering this...