linux-kernel - Re: [PATCH] kvm: x86: Print "disabled by bios" only once per host

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <70b4d8fa-57c0-055b-8391-4952dec32a58@criteo.com>
Date:   Tue, 18 Feb 2020 17:28:49 +0100
From:   Erwan Velu <e.velu@...teo.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Erwan Velu <erwanaliasr1@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        jmattson@...gle.com
Subject: Re: [PATCH] kvm: x86: Print "disabled by bios" only once per host

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,