linux-kernel - Re: [RFC PATCH 2/2] KVM: SVM: Add MSR feature support for serializing LFENCE

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Date:   Wed, 14 Feb 2018 11:08:33 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Tom Lendacky <thomas.lendacky@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [RFC PATCH 2/2] KVM: SVM: Add MSR feature support for serializing
 LFENCE

On 14/02/2018 05:39, Tom Lendacky wrote:
> On 2/13/2018 10:22 AM, Paolo Bonzini wrote:
>> On 08/02/2018 23:58, Tom Lendacky wrote:
>>> Create an entry in the new MSR as a feature framework to allow a guest to
>>> recognize LFENCE as a serializing instruction on AMD processors.  The MSR
>>> can only be set by the host, any write by the guest will be ignored.  A
>>> read by the guest will return the value as set by the host.  In this way,
>>> the support to expose the feature to the guest is controlled by the
>>> hypervisor.
>>>
>>> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
>>> ---
>>>  arch/x86/kvm/svm.c |   16 ++++++++++++++++
>>>  arch/x86/kvm/x86.c |    6 ++++++
>>>  2 files changed, 22 insertions(+)
>>>
>>> @@ -4047,6 +4052,17 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>>>  	case MSR_VM_IGNNE:
>>>  		vcpu_unimpl(vcpu, "unimplemented wrmsr: 0x%x data 0x%llx\n", ecx, data);
>>>  		break;
>>> +	case MSR_F10H_DECFG:
>>> +		/* Only the host can set this MSR, silently ignore */
>>> +		if (!msr->host_initiated)
>>> +			break;
>>
>> Just one thing I'm wondering, should we #GP if the guest attempts to
>> clear MSR_F10H_DECFG_LFENCE_SERIALIZE?
> 
> It would be more consistent with other entries to do "return 1" here
> instead.  The current kernel code that writes this bit is using
> msr_set_bit(), so a #GP is caught and handled.

That's also okay.  We don't know about Windows though...

Paolo