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Date:   Tue, 10 Nov 2020 07:31:51 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     "Luck, Tony" <tony.luck@intel.com>
Cc:     Jim Mattson <jmattson@google.com>, Qian Cai <cai@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-tip-commits@vger.kernel.org" 
        <linux-tip-commits@vger.kernel.org>, x86 <x86@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>
Subject: Re: [PATCH] x86/mce: Check for hypervisor before enabling additional
 error logging
Message-ID: <20201110063151.GB7290@nazgul.tnic>
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On Mon, Nov 09, 2020 at 03:24:02PM -0800, Luck, Tony wrote:
> Booting as a guest under KVM results in error messages about
> unchecked MSR access:
> 
> [    6.814328][    T0] unchecked MSR access error: RDMSR from 0x17f at rIP: 0xffffffff84483f16 (mce_intel_feature_init+0x156/0x270)
> 
> because KVM doesn't provide emulation for random model specific registers.
> 
> Check for X86_FEATURE_HYPERVISOR and skip trying to enable the mode (a
> guest shouldn't be concerned with corrected errors anyway).
> 
> Reported-by: Qian Cai <cai@redhat.com>
> Fixes: 68299a42f842 ("x86/mce: Enable additional error logging on certain Intel CPUs")
> Signed-off-by: Tony Luck <tony.luck@intel.com>
> ---
>  arch/x86/kernel/cpu/mce/intel.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/mce/intel.c b/arch/x86/kernel/cpu/mce/intel.c
> index b47883e364b4..7f7d863400b7 100644
> --- a/arch/x86/kernel/cpu/mce/intel.c
> +++ b/arch/x86/kernel/cpu/mce/intel.c
> @@ -517,6 +517,9 @@ static void intel_imc_init(struct cpuinfo_x86 *c)
>  {
>  	u64 error_control;
>  
> +	if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
> +		return;
> +

Frankly, I'm tired of wagging the dog because the tail can't. If
qemu/kvm can't emulate a CPU model fully then it should ignore those
unknown MSR accesses by default, i.e., that "ignore_msrs" functionality
should be on by default I'd say...

We certainly can't be sprinkling this check everytime the kernel tries
to do something as basic as read an MSR.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette