linux-kernel - Re: [PATCH v5 2/5] KVM: x86: Add IBPB support

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Message-ID: <94a067bd-106a-e683-14e0-9a5617edd0fc@amazon.com>
Date:   Wed, 31 Jan 2018 21:36:24 +0100
From:   KarimAllah Ahmed <karahmed@...zon.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        KarimAllah Ahmed <karahmed@...zon.de>
CC:     <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <x86@...nel.org>, Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Peter Zijlstra" <peterz@...radead.org>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v5 2/5] KVM: x86: Add IBPB support

On 01/31/2018 09:28 PM, Konrad Rzeszutek Wilk wrote:
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index d46a61b..2e4e8af 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -2285,6 +2285,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>   	if (per_cpu(current_vmcs, cpu) != vmx->loaded_vmcs->vmcs) {
>>   		per_cpu(current_vmcs, cpu) = vmx->loaded_vmcs->vmcs;
>>   		vmcs_load(vmx->loaded_vmcs->vmcs);
>> +		indirect_branch_prediction_barrier();
>>   	}
>>   
>>   	if (!already_loaded) {
>> @@ -3342,6 +3343,26 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>   	case MSR_IA32_TSC:
>>   		kvm_write_tsc(vcpu, msr_info);
>>   		break;
>> +	case MSR_IA32_PRED_CMD:
>> +		if (!msr_info->host_initiated &&
>> +		    !guest_cpuid_has(vcpu, X86_FEATURE_IBPB) &&
>> +		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
>> +			return 1;
>> +
>> +		if (data & ~PRED_CMD_IBPB)
>> +			return 1;
>> +
>> +		if (!data)
>> +			break;
>> +
>> +		wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_IBPB);
>> +
>> +		if (is_guest_mode(vcpu))
>> +			break;
> 
> Don't you want this the other way around? That is first do the disable_intercept
> and then add the 'if (is_guest_mode(vcpu))' ? Otherwise the very first
> MSR write from the guest is going to hit condition above and never end
> up executing the disabling of the intercept?

is_guest_mode is checking if this is an L2 guest. I *should not* do
disable_intercept on the L1 guest bitmap if it is an L2 guest that is
why this check happens before disable_intercept.

For the short-circuited L2 path, nested_vmx_merge_msr_bitmap will
properly update the L02 MSR bitmap and use it.

So the checks are fine AFAICT.

> 
>> +
>> +		vmx_disable_intercept_for_msr(vmx->vmcs01.msr_bitmap, MSR_IA32_PRED_CMD,
>> +					      MSR_TYPE_W);
>> +		break;
>>   	case MSR_IA32_CR_PAT:
>>   		if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT) {
>>   			if (!kvm_mtrr_valid(vcpu, MSR_IA32_CR_PAT, data))
> 
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