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Message-ID: <c863ffcfd4edda9a1a46e3351766a655c5523f7d.camel@infradead.org>
Subject: Re: [PATCH 2/2] KVM: selftests: Add KVM/PV clock selftest to prove
 timer drift correction
From: David Woodhouse <dwmw2@infradead.org>
To: "Chen, Zide" <zide.chen@intel.com>, Jack Allister <jalliste@amazon.com>,
  Paolo Bonzini <pbonzini@redhat.com>, Jonathan Corbet <corbet@lwn.net>,
 Sean Christopherson <seanjc@google.com>,  Thomas Gleixner
 <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov
 <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
 "H. Peter Anvin" <hpa@zytor.com>,  Shuah Khan <shuah@kernel.org>
Cc: Paul Durrant <paul@xen.org>, kvm@vger.kernel.org,
 linux-doc@vger.kernel.org,  linux-kernel@vger.kernel.org,
 linux-kselftest@vger.kernel.org
Date: Sat, 20 Apr 2024 17:03:12 +0100
In-Reply-To: <6dca783b-6532-4fa7-9e04-1c0a382a00b0@intel.com>
References: <20240408220705.7637-1-jalliste@amazon.com>
	 <20240408220705.7637-3-jalliste@amazon.com>
	 <3664e8ec-1fa1-48c0-a80d-546b7f6cd671@intel.com>
	 <17F1A2E9-6BAD-40E7-ACDD-B110CFC124B3@infradead.org>
	 <65FF4D51-05A8-42E0-9D07-6E42913CC75E@infradead.org>
	 <6dca783b-6532-4fa7-9e04-1c0a382a00b0@intel.com>
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On Fri, 2024-04-19 at 16:54 -0700, Chen, Zide wrote:
>=20
> However, the selftest hangs:
>=20
> [Apr19 16:15] kselftest: Running tests in kvm
> [Apr19 16:16] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> [=C2=A0 +0.000628] rcu:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 78-...0: (1 GPs beh=
ind) idle=3D3c8c/1/0x4000000000000000 softirq=3D5908/5913 fqs=3D14025
> [=C2=A0 +0.000468] rcu:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 (detected by 104, t=
=3D60003 jiffies, g=3D60073, q=3D3100 ncpus=3D128)
> [=C2=A0 +0.000389] Sending NMI from CPU 104 to CPUs 78:
> [=C2=A0 +0.000360] NMI backtrace for cpu 78
> [=C2=A0 +0.000004] CPU: 78 PID: 33515 Comm: pvclock_test Tainted: G O=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 6.9.0-rc1zide-l0+ #194
> [=C2=A0 +0.000003] Hardware name: Inspur NF5280M7/NF5280M7, BIOS 05.08.01=
 08/18/2023
> [=C2=A0 +0.000002] RIP: 0010:pvclock_update_vm_gtod_copy+0xb5/0x200 [kvm]

Ah, kvm_get_time_scale() doesn't much like being asked to scale to zero.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a07b60351894..45fb99986cf9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3046,7 +3046,8 @@ static void pvclock_update_vm_gtod_copy(struct kvm *k=
vm)
 		 * Copy from the field protected solely by ka->tsc_write_lock,
 		 * to the field protected by the ka->pvclock_sc seqlock.
 		 */
-		ka->master_tsc_scaling_ratio =3D ka->last_tsc_scaling_ratio;
+		ka->master_tsc_scaling_ratio =3D ka->last_tsc_scaling_ratio ? :
+			kvm_caps.default_tsc_scaling_ratio;
=20
 		/*
 		 * Calculate the scaling factors precisely the same way


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