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Date: Tue, 23 Apr 2024 22:02:40 +0100
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
Subject: =?US-ASCII?Q?Re=3A_=5BPATCH_2/2=5D_KVM=3A_selftests=3A_Add_KVM/PV_c?=
 =?US-ASCII?Q?lock_selftest_to_prove_timer_drift_correction?=
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On 23 April 2024 18:59:21 BST, "Chen, Zide" <zide=2Echen@intel=2Ecom> wrote=
:
>
>
>On 4/23/2024 12:49 AM, David Woodhouse wrote:
>>> 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 fo=
r
>>> 1000+ iterations, w/ or w/o TSC scaling, the TEST_ASSERT(delta_correct=
ed
>>> =C2=A0<=3D =C2=B11) never got hit=2E This is awesome!
>>>
>>> However, without KVM_REQ_GLOBAL_CLOCK_UPDATE, it still fails on creati=
ng
>>> a VM=2E Maybe the init sequence sill needs some rework=2E
>>=20
>> That one confuses me=2E The crash is actually in debugfs, as it's
>> registering the per-vm or per-vcpu stats=2E I can't imagine *how* that'=
s
>> occurring=2E Or see why the availability of TSC scaling would cause it =
to
>> show up for you and not me=2E Can I have your =2Econfig please?
>>=20
>> First thought would be that there's some change in the KVM structures
>> and you have some stale object files using the old struct, but then I
>> realise I forgot to actually *remove* the now-unused
>> kvmclock_update_work from x86's struct kvm_arch anyway=2E
>>=20
>> I'll try to reproduce, as I think I want to *know* what's going on
>> here, even if I am going to drop that patch as mentioned in=C2=A0
>> https://lore=2Ekernel=2Eorg/kvm/a6723ac9e0169839cb33e8022a47c2de213866a=
c=2Ecamel@infradead=2Eorg
>>=20
>> Are you able to load that vmlinux in gdb and
>> (gdb) list *start_creating+0x80
>> (gdb) list *kvm_create_vm_debugfs+0x28b
>>=20
>> Thanks again=2E
>
>My apologies, it turns out the KVM_REQ_GLOBAL_CLOCK_UPDATE is not
>needed=2E Today I can't reproduce the issue after removing it=2E  Yesterd=
ay
>I thought it may miss something related to pfncache=2E
>
>To be clear, with the above mentioned change to
>kvm_scale_tsc(master_tsc_scaling_ratio), the selftest runs reliably
>regardless TSC scaling is enabled or not=2E

Thanks=2E That version is now in my git tree and I have tested it myself o=
n Skylake=2E Then I got distracted by reverse-engineering kvm_get_time_scal=
e() so I could actually add some comments to it=2E

I'm still going to have to put the clock updates back though, for the non-=
masterclock case=2E

While I'm ripping all this up I guess I ought to rename it to "reference c=
lock" too?