linux-kernel - Re: [PATCH] KVM: x86: svm: report MSR_IA32_MCG_EXT

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Date:   Fri, 21 Dec 2018 11:06:52 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Joerg Roedel <joro@...tes.org>, x86@...nel.org
Subject: Re: [PATCH] KVM: x86: svm: report MSR_IA32_MCG_EXT_CTL as unsupported

Radim Krčmář <rkrcmar@...hat.com> writes:

> 2018-12-19 12:06+0100, Vitaly Kuznetsov:
>> AMD doesn't seem to implement MSR_IA32_MCG_EXT_CTL and svm code in kvm
>> knows nothing about it, however, this MSR is among emulated_msrs and
>> thus returned with KVM_GET_MSR_INDEX_LIST. The consequent KVM_GET_MSRS,
>> of course, fails.
>> 
>> Report the MSR as unsupported to not confuse userspace.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  arch/x86/kvm/svm.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>> 
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index 2acb42b74a51..dfdf7d0b7f88 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -5845,6 +5845,13 @@ static bool svm_cpu_has_accelerated_tpr(void)
>>  
>>  static bool svm_has_emulated_msr(int index)
>>  {
>> +	switch (index) {
>> +	case MSR_IA32_MCG_EXT_CTL:
>> +		return false;
>> +	default:
>> +		break;
>
> Queued, thanks.
>
> Btw, I would prefer this without the
>
>   default: break;
>
> as I don't think we'll ever add something there.

"640K ought to be enough for anybody" :-)

But in case you commit it as

return index != MSR_IA32_MCG_EXT_CTL; 

I won't object. Thanks,

-- 
Vitaly