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

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Date:   Tue, 21 Sep 2021 13:50:09 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ashish Kalra <ashish.kalra@....com>
Cc:     Steve Rutherford <srutherford@...gle.com>, pbonzini@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        joro@...tes.org, bp@...en8.de, 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, Ashish Kalra wrote:
> This is simply a Hack, i don't think this is a good approach to take forward.

But a clever hack ;-)

> > 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.  
> 
> Now, as i mentioned earlier, apply_alternatives() is only called boot
> CPU identification has been done which is a lot of support code which
> may be dependent on earlier setup_arch() code and then it does CPU
> mitigtion selections before patching alternatives, again which may have
> dependencies on previous code paths in setup_arch(), so i am not sure if
> we can call apply_alternatives() earlier. 

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.

> Maybe for a guest kernel and virtualized boot enviroment, CPU
> identification may not be as complicated as for a physical host, but
> still it may have dependencies on earlier architecture specific boot
> code.
> 
> > 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.
> 
> If we have to do this in the future and as Sean mentioned ealier that
> vmcall needs to be fixed for TDX (as it will cause a #VE), then why not
> add this abstraction right now ?

I'm not objecting to adding a PV op, I'm objecting to kvm_sev_hypercall3().  If
others disagree and feel it's the way forward, I certainly won't stand in the way,
but IMO it's unnecessary code duplication.