linux-kernel - Re: VMX Preemption Timer appears to be buggy on SKX, CLX, and ICX

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Message-ID: <9b19f440-ee78-4a4f-ab87-e9fff26ea6a4@intel.com>
Date: Wed, 3 Jul 2024 14:37:37 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: <isaku.yamahata@...el.com>, <pbonzini@...hat.com>,
	<erdemaktas@...gle.com>, <vkuznets@...hat.com>, <vannapurve@...gle.com>,
	<jmattson@...gle.com>, <mlevitsk@...hat.com>, <xiaoyao.li@...el.com>,
	<chao.gao@...el.com>, <rick.p.edgecombe@...el.com>, <yuan.yao@...el.com>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, "Hao, Xudong"
	<xudong.hao@...el.com>
Subject: Re: VMX Preemption Timer appears to be buggy on SKX, CLX, and ICX


(a short update ...)

On 7/3/24 1:14 PM, Reinette Chatre wrote:
> On 6/28/24 5:39 PM, Sean Christopherson wrote:
>> Forking this off to try and avoid confusion...
>>
>> On Wed, Jun 12, 2024, Reinette Chatre wrote:
> 
> ...
> 
>>> +
>>> +        freq = (tmict - tmcct) * tdcrs[i].divide_count * tsc_hz / (tsc1 - tsc0);
>>> +        /* Check if measured frequency is within 1% of configured frequency. */
>>> +        GUEST_ASSERT(freq < apic_hz * 101 / 100);
>>> +        GUEST_ASSERT(freq > apic_hz * 99 / 100);
>>> +    }
>>
>> This test fails on our SKX, CLX, and ICX systems due to what appears to be a CPU
>> bug.  It looks like something APICv related is clobbering internal VMX timer state?
>> Or maybe there's a tearing or truncation issue?
> 
> It has been a few days. Just a note to let you know that we are investigating this.
> On my side I have not yet been able to reproduce this issue. I tested
> kvm-x86-next-2024.06.28 on an ICX and an CLX system by running 100 iterations of
> apic_bus_clock_test and they all passed. Since I have lack of experience here there are
> some Intel virtualization experts helping out with this investigation and I hope that
> they will be some insights from the analysis and testing that you already provided.

I have now been able to test on SKX also and I am not yet able to reproduce. For
reference, the systems I tested on are:
SKX: https://ark.intel.com/content/www/us/en/ark/products/120507/intel-xeon-platinum-8170m-processor-35-75m-cache-2-10-ghz.html
ICX: https://ark.intel.com/content/www/us/en/ark/products/212459/intel-xeon-platinum-8360y-processor-54m-cache-2-40-ghz.html
CLX: https://ark.intel.com/content/www/us/en/ark/products/192476/intel-xeon-platinum-8260l-processor-35-75m-cache-2-40-ghz.html

Reinette