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

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Date:   Fri, 14 Feb 2020 09:05:08 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Erwan Velu <erwanaliasr1@...il.com>
Cc:     Erwan Velu <e.velu@...teo.com>,
        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
Subject: Re: [PATCH] kvm: x86: Print "disabled by bios" only once per host

On Fri, Feb 14, 2020 at 03:30:35PM +0100, Erwan Velu wrote:
> The current behavior is to print a "disabled by bios" message per CPU thread.
> As modern CPUs can have up to 64 cores, 128 on a dual socket, and turns this
> printk to be a pretty noisy by showing up to 256 times the same line in a row.
> 
> This patch offer to only print the message once per host considering the BIOS will
> disabled the feature for all sockets/cores at once and not on a per core basis.

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.

[*] https://lkml.kernel.org/r/20190826182320.9089-1-tony.luck@intel.com

> Signed-off-by: Erwan Velu <e.velu@...teo.com>
> ---
>  arch/x86/kvm/x86.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fbabb2f06273..8f0d7a09d453 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7300,7 +7300,7 @@ int kvm_arch_init(void *opaque)
>  		goto out;
>  	}
>  	if (ops->disabled_by_bios()) {
> -		printk(KERN_ERR "kvm: disabled by bios\n");
> +		printk_once(KERN_ERR "kvm: disabled by bios\n");
>  		r = -EOPNOTSUPP;
>  		goto out;
>  	}
> -- 
> 2.24.1
>