linux-kernel - [PATCH] KVM: SVM: Hide SEV migration lockdep goo behind CONFIG_DEBUG_LOCK

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Date:   Mon, 13 Jun 2022 21:42:37 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Tom Rix <trix@...hat.com>,
        kvm@...r.kernel.org, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, Peter Gonda <pgonda@...gle.com>
Subject: [PATCH] KVM: SVM: Hide SEV migration lockdep goo behind CONFIG_DEBUG_LOCK_ALLOC

Wrap the manipulation of @role and the manual mutex_{release,acquire}()
invocations in CONFIG_PROVE_LOCKING=y to squash a clang-15 warning.  When
building with -Wunused-but-set-parameter and CONFIG_DEBUG_LOCK_ALLOC=n,
clang-15 seees there's no usage of @role in mutex_lock_killable_nested()
and yells.  PROVE_LOCKING selects DEBUG_LOCK_ALLOC, and the only reason
KVM manipulates @role is to make PROVE_LOCKING happy.

To avoid true ugliness, use "i" and "j" to detect the first pass in the
loops; the "idx" field that's used by kvm_for_each_vcpu() is guaranteed
to be '0' on the first pass as it's simply the first entry in the vCPUs
XArray, which is fully KVM controlled.  kvm_for_each_vcpu() passes '0'
for xa_for_each_range()'s "start", and xa_for_each_range() will not enter
the loop if there's no entry at '0'.

Fixes: 0c2c7c069285 ("KVM: SEV: Mark nested locking of vcpu->lock")
Reported-by: kernel test robot <lkp@...el.com>
Cc: Peter Gonda <pgonda@...gle.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---

Compile tested only, still haven't figured out why SEV is busted on our
test systems with upstream kernels.  I also haven't verified this squashes
the clang-15 warning, so a thumbs up on that end would be helpful.

 arch/x86/kvm/svm/sev.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 51fd985cf21d..309bcdb2f929 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -1606,38 +1606,35 @@ static int sev_lock_vcpus_for_migration(struct kvm *kvm,
 {
 	struct kvm_vcpu *vcpu;
 	unsigned long i, j;
-	bool first = true;

 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		if (mutex_lock_killable_nested(&vcpu->mutex, role))
 			goto out_unlock;

-		if (first) {
+#ifdef CONFIG_PROVE_LOCKING
+		if (!i)
 			/*
 			 * Reset the role to one that avoids colliding with
 			 * the role used for the first vcpu mutex.
 			 */
 			role = SEV_NR_MIGRATION_ROLES;
-			first = false;
-		} else {
+		else
 			mutex_release(&vcpu->mutex.dep_map, _THIS_IP_);
-		}
+#endif
 	}

 	return 0;

 out_unlock:

-	first = true;
 	kvm_for_each_vcpu(j, vcpu, kvm) {
 		if (i == j)
 			break;

-		if (first)
-			first = false;
-		else
+#ifdef CONFIG_PROVE_LOCKING
+		if (j)
 			mutex_acquire(&vcpu->mutex.dep_map, role, 0, _THIS_IP_);
-
+#endif

 		mutex_unlock(&vcpu->mutex);
 	}

base-commit: 8baacf67c76c560fed954ac972b63e6e59a6fba0
-- 
2.36.1.476.g0c4daa206d-goog