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From: Peter Gonda <pgonda@google.com>
To: Marc Orr <marcorr@google.com>
Cc: kvm list <kvm@vger.kernel.org>,
	Sean Christopherson <seanjc@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	David Rientjes <rientjes@google.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2 V4] KVM, SEV: Add support for SEV intra host migration
Date: Thu, 19 Aug 2021 15:00:25 -0600	[thread overview]
Message-ID: <CAMkAt6oJcW3MHP3fod9RnRHCEYp-whdEtBTyfuqgFgATKa=3Hg@mail.gmail.com> (raw)
In-Reply-To: <CAA03e5Gh0kJYHP1R3F7uh6x83LBFPp=af2xt7q3epgg+8XW53g@mail.gmail.com>

> >
> > +static int svm_sev_lock_for_migration(struct kvm *kvm)
> > +{
> > +       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > +       int ret;
> > +
> > +       /*
> > +        * Bail if this VM is already involved in a migration to avoid deadlock
> > +        * between two VMs trying to migrate to/from each other.
> > +        */
> > +       spin_lock(&sev->migration_lock);
> > +       if (sev->migration_in_progress)
> > +               ret = -EBUSY;
> > +       else {
> > +               /*
> > +                * Otherwise indicate VM is migrating and take the KVM lock.
> > +                */
> > +               sev->migration_in_progress = true;
> > +               mutex_lock(&kvm->lock);
> > +               ret = 0;
> > +       }
> > +       spin_unlock(&sev->migration_lock);
> > +
> > +       return ret;
> > +}
> > +
> > +static void svm_unlock_after_migration(struct kvm *kvm)
> > +{
> > +       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > +
> > +       mutex_unlock(&kvm->lock);
> > +       WRITE_ONCE(sev->migration_in_progress, false);
> > +}
> > +
>
> This entire locking scheme seems over-complicated to me. Can we simply
> rely on `migration_lock` and get rid of `migration_in_progress`? I was
> chatting about these patches with Peter, while he worked on this new
> version. But he mentioned that this locking scheme had been suggested
> by Sean in a previous review. Sean: what do you think? My rationale
> was that this is called via a VM-level ioctl. So serializing the
> entire code path on `migration_lock` seems fine. But maybe I'm missing
> something?
>

Marc I think that only having the spin lock could result in
deadlocking. If userspace double migrated 2 VMs, A and B for
discussion, A could grab VM_A.spin_lock then VM_A.kvm_mutex. Meanwhile
B could grab VM_B.spin_lock and VM_B.kvm_mutex. Then A attempts to
grab VM_B.spin_lock and we have a deadlock. If the same happens with
the proposed scheme when A attempts to lock B, VM_B.spin_lock will be
open but the bool will mark the VM under migration so A will unlock
and bail. Sean originally proposed a global spin lock but I thought a
per kvm_sev_info struct would also be safe.

  reply	other threads:[~2021-08-19 21:00 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-19 15:49 [PATCH 0/2 V4] Add AMD SEV and SEV-ES intra host migration support Peter Gonda
2021-08-19 15:49 ` [PATCH 1/2 V4] KVM, SEV: Add support for SEV intra host migration Peter Gonda
2021-08-19 16:23   ` Marc Orr
2021-08-19 21:00     ` Peter Gonda [this message]
2021-08-19 22:58       ` Sean Christopherson
2021-08-20  6:35         ` Marc Orr
2021-08-20 14:50           ` Sean Christopherson
2021-08-20 20:53         ` Marc Orr
2021-08-23 16:39           ` Peter Gonda
2021-08-19 15:49 ` [PATCH 2/2 V4] KVM, SEV: Add support for SEV-ES " Peter Gonda
2021-08-20 21:00   ` Marc Orr
2021-08-23 16:38     ` Peter Gonda

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