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

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Message-ID: <9412F330-145D-4319-86E1-D5C5FAFBBF9D@infradead.org>
Date: Sat, 20 Apr 2024 11:32:38 +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 20 April 2024 00:54:05 BST, "Chen, Zide" <zide.chen@...el.com> wrote:
>
>
>On 4/19/2024 11:43 AM, David Woodhouse wrote:
>> On 19 April 2024 19:40:06 BST, David Woodhouse <dwmw2@...radead.org> wrote:
>>> On 19 April 2024 18:13:16 BST, "Chen, Zide" <zide.chen@...el.com> wrote:
>>>> I'm wondering what's the underling theory that we definitely can achieve
>>>> ±1ns accuracy? I tested it on a Sapphire Rapids @2100MHz TSC frequency,
>>>> and I can see delta_corrected=2 in ~2% cases.
>>>
>>> Hm. Thanks for testing!
>>>
>>> So the KVM clock is based on the guest TSC. Given a delta between the guest TSC T and some reference point in time R, the KVM clock is expressed as a(T-R)+r, where little r is the value of the KVM clock when the guest TSC was R, and (a) is the rate of the guest TSC.
>>>
>>> When set the clock with KVM_SET_CLOCK_GUEST, we are changing the values of R and r to a new point in time. Call the new ones Q and q respectively.
>>>
>>> But we calculate precisely (within 1ns at least) what the KVM clock would have been with the *old* formula, and adjust our new offset (q) so that at our new reference TSC value Q, the formulae give exactly the same result.
>>>
>>> And because the *rates* are the same, they should continue to give the same results, ±1ns.
>>>
>>> Or such *was* my theory, at least. 
>>>
>>> Would be interesting to see it disproven with actual numbers for the old+new pvclock structs, so I can understand where the logic goes wrong.
>>>
>>> Were you using frequency scaling?
>>>
>> 
>> Oh, also please could you test the updated version I posted yesterday, from https://git.infradead.org/?p=users/dwmw2/linux.git;a=shortlog;h=refs/heads/clocks
>
>I failed to check out your branch, instead I downloaded the patch series
>from:
>https://lore.kernel.org/linux-kselftest/FABCFBD0-4B76-4662-9F7B-7E1A856BBBB6@infradead.org/T/#t
>
>However, the selftest hangs:

Odd. It locks up in kvm_arch_init_vm(). Maybe when I get back to my desk something will be obvious. But please could I have your .config?

If you're able to bisect and see which patch causes that, it would also be much appreciated. Thanks!

>[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]
>[  +0.000079] Code: ea 83 e1 40 48 0f 45 c2 31 d2 48 3d 00 94 35 77 76
>0e 48 d1 e8 83 ea 01 48 3d 00 94 35 77 77 f2 48 3d 00 ca 9a 3b 89 c1 77
>0d <01> c9 83 c2 01 81 f9 00 ca 9a 3b 76 f3 88 93 8c 95 00 00 31 c0 ba
>[  +0.000002] RSP: 0018:ff368a58cfe07e30 EFLAGS: 00000087
>[  +0.000002] RAX: 0000000000000000 RBX: ff368a58e0ccd000 RCX:
>0000000000000000
>[  +0.000001] RDX: 000000005ca49a49 RSI: 00000000000029aa RDI:
>0000019ee77a1c00
>[  +0.000002] RBP: ff368a58cfe07e50 R08: 0000000000000001 R09:
>0000000000000000
>[  +0.000000] R10: ff26383d853ab400 R11: 0000000000000002 R12:
>0000000000000000
>[  +0.000001] R13: ff368a58e0cd6400 R14: 0000000000000293 R15:
>ff368a58e0cd69f0
>[  +0.000001] FS:  00007f6946473740(0000) GS:ff26384c7fb80000(0000)
>knlGS:0000000000000000
>[  +0.000001] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>[  +0.000001] CR2: 00007f69463bd445 CR3: 000000016f466006 CR4:
>0000000000f71ef0
>[  +0.000001] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
>0000000000000000
>[  +0.000000] DR3: 0000000000000000 DR6: 00000000fffe07f0 DR7:
>0000000000000400
>[  +0.000001] PKRU: 55555554
>[  +0.000001] Call Trace:
>[  +0.000004]  <NMI>
>[  +0.000003]  ? nmi_cpu_backtrace+0x87/0xf0
>[  +0.000008]  ? nmi_cpu_backtrace_handler+0x11/0x20
>[  +0.000005]  ? nmi_handle+0x5f/0x170
>[  +0.000005]  ? pvclock_update_vm_gtod_copy+0xb5/0x200 [kvm]
>[  +0.000045]  ? default_do_nmi+0x79/0x1a0
>[  +0.000004]  ? exc_nmi+0xf0/0x130
>[  +0.000001]  ? end_repeat_nmi+0xf/0x53
>[  +0.000006]  ? pvclock_update_vm_gtod_copy+0xb5/0x200 [kvm]
>[  +0.000041]  ? pvclock_update_vm_gtod_copy+0xb5/0x200 [kvm]
>[  +0.000040]  ? pvclock_update_vm_gtod_copy+0xb5/0x200 [kvm]
>[  +0.000039]  </NMI>
>[  +0.000000]  <TASK>
>[  +0.000001]  ? preempt_count_add+0x73/0xa0
>[  +0.000004]  kvm_arch_init_vm+0xf1/0x1e0 [kvm]
>[  +0.000049]  kvm_create_vm+0x370/0x650 [kvm]
>[  +0.000036]  kvm_dev_ioctl+0x88/0x180 [kvm]
>[  +0.000034]  __x64_sys_ioctl+0x8e/0xd0
>[  +0.000007]  do_syscall_64+0x5b/0x120
>[  +0.000003]  entry_SYSCALL_64_after_hwframe+0x6c/0x74
>[  +0.000003] RIP: 0033:0x7f694631a94f
>[  +0.000002] 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 77 1f 48 8b 44 24 18 64 48 2b 04 25 28 00
>[  +0.000001] RSP: 002b:00007ffca91b2e50 EFLAGS: 00000246 ORIG_RAX:
>0000000000000010
>[  +0.000002] RAX: ffffffffffffffda RBX: 0000000000434480 RCX:
>00007f694631a94f
>[  +0.000001] RDX: 0000000000000000 RSI: 000000000000ae01 RDI:
>0000000000000005
>[  +0.000000] RBP: 0000000000000009 R08: 000000000041b198 R09:
>000000000041bfbf
>[  +0.000001] R10: 00007f69463d8882 R11: 0000000000000246 R12:
>0000000000434480
>[  +0.000000] R13: 000000000041e0f0 R14: 0000000000001000 R15:
>0000000000000207
>[  +0.000002]  </TASK>