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From:   Peter Gonda <pgonda@google.com>
Date:   Mon, 4 Apr 2022 15:51:27 -0600
Message-ID: <CAMkAt6r+DpuFcnhVMcLnj2tEDmefdCnwUP7axdLiRomCVmdjXg@mail.gmail.com>
Subject: Re: [PATCH] KVM: SEV: Mark nested locking of vcpu->lock
To:     Sean Christopherson <seanjc@google.com>
Cc:     kvm list <kvm@vger.kernel.org>,
        John Sperbeck <jsperbeck@google.com>,
        David Rientjes <rientjes@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>
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>
> This is rather gross, and I'm guessing it adds extra work for the non-lockdep
> case, assuming the compiler isn't so clever that it can figure out that the result
> is never used.  Not that this is a hot path...
>
> Does each lock actually need a separate subclass?  If so, why don't the other
> paths that lock all vCPUs complain?
>
> If differentiating the two VMs is sufficient, then we can pass in SINGLE_DEPTH_NESTING
> for the second round of locks.  If a per-vCPU subclass is required, we can use the
> vCPU index and assign evens to one and odds to the other, e.g. this should work and
> compiles to a nop when LOCKDEP is disabled (compile tested only).  It's still gross,
> but we could pretty it up, e.g. add defines for the 0/1 param.

I checked and the perf vCPU subclassing is required. If I just only
use a SINGLE_DEPTH_NESTING on the second VM's vCPUs I still see the
warning.

This odds and evens approach seems much better. I'll update to use
that in the V2 unless there is a better idea.