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Subject: Re: [PATCH] kvm: x86: Print "disabled by bios" only once per host
To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Erwan Velu <erwanaliasr1@gmail.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        jmattson@google.com
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 <20200214170508.GB20690@linux.intel.com>
From:   Erwan Velu <e.velu@criteo.com>
Message-ID: <70b4d8fa-57c0-055b-8391-4952dec32a58@criteo.com>
Date:   Tue, 18 Feb 2020 17:28:49 +0100
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On 14/02/2020 18:05, Sean Christopherson wrote:
> This has come up before[*].  Using _once() doesn't fully solve the issue
> when KVM is built as a module.  The spam is more than likely a userspace
> bug, i.e. userspace is probing KVM on every CPU.

I made some progress on this.


That's "/usr/bin/udevadm trigger --type=devices --action=add" the culprit.

It does echo "add" in /sys/devices/system/cpu/cpu<x>/uevent

For the each cpu, it does the 'add' which trigger the "disabled by bios" 
message from kvm_arch_init.

Note that doing a "add" on the same processor will trigger the same 
message at every "add" event.


So I tried the patch of using pr_err_once() instead of printk() and the 
behavior is fine : despite the number of "add" generated, there is a 
single line being printed out.

Without the patch, every "add" generates the "disabled by bios" message.


So the question is : do we want to handle the case where a possible bios 
missed the configuration of some cores ?

If no, then the patch is fine and could be submitted. I don't see the 
need of printing this message at every call as it pollute the kernel log.

If yes, then we need to keep a trace of the number of enabled/disabled 
cores so we can report a mismatch. As this message seems printed per 
cpu, that would kind of mean a global variable right ?


What are your recommendations on this ?


Erwan,