linux-kernel - Re: [RFC] KVM: x86: Support KVM VMs sharing SEV context

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Date:   Tue, 16 Mar 2021 11:08:34 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Nathan Tempelman <natet@...gle.com>,
        Thomas Lendacky <thomas.lendacky@....com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Steve Rutherford <srutherford@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Ashish Kalra <Ashish.Kalra@....com>
Subject: Re: [RFC] KVM: x86: Support KVM VMs sharing SEV context

On Tue, Mar 16, 2021, Paolo Bonzini wrote:
> On 16/03/21 18:52, Sean Christopherson wrote:
> > > I don't
> > > know that holding the fd instead of the kvm makes that much better though,
> > > are there advantages to that I'm not seeing?
> > If there's no kvm pointer, it's much more difficult for someone to do the wrong
> > thing, and any such shenanigans stick out like a sore thumb in patches, which
> > makes reviewing future changes easier.
> 
> On the other hand holding the fd open complicates the code, reference
> counting rules are already hard enough.

How so?  KVM already has to do "fget(source_kvm)", can't we just hold onto to
that instead of doing an additional kvm_get_kvm()?