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From:   Paolo Bonzini <pbonzini@redhat.com>
Date:   Fri, 29 Apr 2022 19:21:33 +0200
Message-ID: <CABgObfa0ubOwNv2Vi9ziEjHXQMR_Sa6P-fwuXfPq76qy0N61kA@mail.gmail.com>
Subject: Re: [PATCH v3] KVM: SEV: Mark nested locking of vcpu->lock
To:     Peter Gonda <pgonda@google.com>
Cc:     John Sperbeck <jsperbeck@google.com>,
        kvm list <kvm@vger.kernel.org>,
        David Rientjes <rientjes@google.com>,
        Sean Christopherson <seanjc@google.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Apr 29, 2022 at 7:12 PM Peter Gonda <pgonda@google.com> wrote:
> Sounds good. Instead of doing this prev_vcpu solution we could just
> keep the 1st vcpu for source and target. I think this could work since
> all the vcpu->mutex.dep_maps do not point to the same string.
>
> Lock:
>          bool acquired = false;
>          kvm_for_each_vcpu(...) {
>                  if (mutex_lock_killable_nested(&vcpu->mutex, role)
>                      goto out_unlock;
>                 acquired = true;
>                  if (acquired)
>                       mutex_release(&vcpu->mutex, role)
>          }

Almost:

          bool first = true;
          kvm_for_each_vcpu(...) {
                  if (mutex_lock_killable_nested(&vcpu->mutex, role)
                      goto out_unlock;
                  if (first)
                      ++role, first = false;
                 else
                      mutex_release(&vcpu->mutex, role);
         }

and to unlock:

          bool first = true;
          kvm_for_each_vcpu(...) {
                if (first)
                      first = false;
                else
                      mutex_acquire(&vcpu->mutex, role);
                mutex_unlock(&vcpu->mutex);
                acquired = false;
          }

because you cannot use the first vCPU's role again when locking.

Paolo