linux-kernel - Re: [PATCH 2/2] KVM: selftests: Add KVM/PV clock selftest to prove timer drift correction

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Message-ID: <c863ffcfd4edda9a1a46e3351766a655c5523f7d.camel@infradead.org>
Date: Sat, 20 Apr 2024 17:03:12 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: "Chen, Zide" <zide.chen@...el.com>, Jack Allister <jalliste@...zon.com>,
  Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
 Sean Christopherson <seanjc@...gle.com>,  Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov
 <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>,  Shuah Khan <shuah@...nel.org>
Cc: Paul Durrant <paul@....org>, kvm@...r.kernel.org,
 linux-doc@...r.kernel.org,  linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: selftests: Add KVM/PV clock selftest to prove
 timer drift correction

On Fri, 2024-04-19 at 16:54 -0700, Chen, Zide wrote:
> 
> However, the selftest hangs:
> 
> [Apr19 16:15] kselftest: Running tests in kvm
> [Apr19 16:16] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> [  +0.000628] rcu:      78-...0: (1 GPs behind) idle=3c8c/1/0x4000000000000000 softirq=5908/5913 fqs=14025
> [  +0.000468] rcu:      (detected by 104, t=60003 jiffies, g=60073, q=3100 ncpus=128)
> [  +0.000389] Sending NMI from CPU 104 to CPUs 78:
> [  +0.000360] NMI backtrace for cpu 78
> [  +0.000004] CPU: 78 PID: 33515 Comm: pvclock_test Tainted: G O       6.9.0-rc1zide-l0+ #194
> [  +0.000003] Hardware name: Inspur NF5280M7/NF5280M7, BIOS 05.08.01 08/18/2023
> [  +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 *kvm)
 		 * 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 = ka->last_tsc_scaling_ratio;
+		ka->master_tsc_scaling_ratio = ka->last_tsc_scaling_ratio ? :
+			kvm_caps.default_tsc_scaling_ratio;

 		/*
 		 * Calculate the scaling factors precisely the same way

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