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

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Date: Mon, 22 Apr 2024 15:02:50 -0700
From: "Chen, Zide" <zide.chen@...el.com>
To: David Woodhouse <dwmw2@...radead.org>, 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 4/20/2024 9:03 AM, David Woodhouse wrote:
> 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
> 		 * that kvm_guest_time_update() does.
>  		last_tsc_hz = kvm_scale_tsc(tsc_khz * 1000,
>                                           ka->last_tsc_scaling_ratio);

Should be ka->master_tsc_scaling_ratio?

If I restored the KVM_REQ_GLOBAL_CLOCK_UPDATE request from
kvm_arch_vcpu_load(), the selftest works for me, and I ran the test for
1000+ iterations, w/ or w/o TSC scaling, the TEST_ASSERT(delta_corrected
 <= ±1) never got hit. This is awesome!

However, without KVM_REQ_GLOBAL_CLOCK_UPDATE, it still fails on creating
a VM. Maybe the init sequence sill needs some rework.

 BUG: unable to handle page fault for address: 005b29e3f221ccf0
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 0
 Oops: 0000 [#1] PREEMPT SMP NOPTI
 CPU: 86 PID: 4118 Comm: pvclock_test Tainted
 Hardware name: Inspur NF5280M7/NF5280M7, BIOS 05.08.01 08/18/2023
 RIP: 0010:start_creating+0x80/0x190
 Code: ce ad 48 c7 c6 70 a1 ce ad 48 c7 c7 80 1c 9b ab e8 b5 10 d5 ff 4c
63 e0 45 85 e4 0f 85 cd 00 00 00 48 85 db 0f 84 b5 00 00 00 <48> 8b 43
30 48 8d b8 b8 >
 RSP: 0018:ff786eaacf3cfdd0 EFLAGS: 00010206
 RAX: 0000000000000000 RBX: 005b29e3f221ccc0 RCX: 0000000000000000
 RDX: 0000000000000001 RSI: ffffffffadcea170 RDI: 0000000000000000
 RBP: ffffffffc06ac8cf R08: ffffffffa6ea0fe0 R09: ffffffffc06a5940
 R10: ff786eaacf3cfe30 R11: 00000013a7b5feaa R12: 0000000000000000
 R13: 0000000000000124 R14: ff786eaacfa11000 R15: 00000000000081a4
 FS:  00007f0837c89740(0000) GS:ff4f44b6bfd80000(0000)
knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0
 CR2: 005b29e3f221ccf0 CR3: 000000014bdf8002 CR4: 0000000000f73ef0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe07f0 DR7: 0000000000000400
 PKRU: 55555554
 Call Trace:
  <TASK>
  ? __die+0x24/0x70
  ? page_fault_oops+0x81/0x150
  ? do_user_addr_fault+0x64/0x6c0
  ? exc_page_fault+0x8a/0x1a0
  ? asm_exc_page_fault+0x26/0x30
  ? start_creating+0x80/0x190
  __debugfs_create_file+0x43/0x1f0
  kvm_create_vm_debugfs+0x28b/0x2d0 [kvm]
  kvm_create_vm+0x457/0x650 [kvm]
  kvm_dev_ioctl+0x88/0x180 [kvm]
  __x64_sys_ioctl+0x8e/0xd0
  do_syscall_64+0x5b/0x120
  entry_SYSCALL_64_after_hwframe+0x71/0x79
 RIP: 0033:0x7f0837b1a94f
 Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89
44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <41> 89 c0
3d 00 f0 ff ff >
 RSP: 002b:00007ffe01be3fc0 EFLAGS: 00000246 ORIG_RAX
 RAX: ffffffffffffffda RBX: 0000000000434480 RCX: 00007f0837b1a94f
 RDX: 0000000000000000 RSI: 000000000000ae01 RDI: 0000000000000005
 RBP: 0000000000000009 R08: 000000000041b1a0 R09: 000000000041bfcf
 R10: 00007f0837bd8882 R11: 0000000000000246 R12: 0000000000434480
 R13: 000000000041e0f0 R14: 0000000000001000 R15: 0000000000000207
  </TASK>
 Modules linked in: kvm_intel(O) kvm(O) [last unloaded: kvm(O)]
 CR2: 005b29e3f221ccf0