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

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Date:   Tue, 21 Sep 2021 16:07:03 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Ashish Kalra <ashish.kalra@....com>,
        Steve Rutherford <srutherford@...gle.com>, pbonzini@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        joro@...tes.org, thomas.lendacky@....com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        brijesh.singh@....com, dovmurik@...ux.ibm.com, tobin@...ux.ibm.com,
        jejb@...ux.ibm.com, dgilbert@...hat.com
Subject: Re: [PATCH v6 1/5] x86/kvm: Add AMD SEV specific Hypercall3

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...