linux-kernel - Re: [PATCH v4 24/25] KVM: VMX: Cache MSR_IA32_VMX_MISC in vmcs

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Date:   Thu, 21 Jul 2022 23:06:01 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 24/25] KVM: VMX: Cache MSR_IA32_VMX_MISC in vmcs_config

On Thu, Jul 14, 2022, Vitaly Kuznetsov wrote:
> @@ -2613,6 +2614,8 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
>  	if (((vmx_msr_high >> 18) & 15) != 6)
>  		return -EIO;
>  
> +	rdmsrl(MSR_IA32_VMX_MISC, misc_msr);

Might make sense to sanitize fields that KVM doesn't use and that are not exposed
to L1.  Not sure it's worthwhile though as many of the bits fall into a grey area,
e.g. all the SMM stuff isn't technically used by KVM, but that's largely because
much of it just isn't relevant to virtualization.

I'm totally ok leaving it as-is, though maybe name it "unsanitized_misc" or so
to make that obvious?

>  	vmcs_conf->size = vmx_msr_high & 0x1fff;
>  	vmcs_conf->basic_cap = vmx_msr_high & ~0x1fff;
>  
> @@ -2624,6 +2627,7 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
>  	vmcs_conf->cpu_based_3rd_exec_ctrl = _cpu_based_3rd_exec_control;
>  	vmcs_conf->vmexit_ctrl         = _vmexit_control;
>  	vmcs_conf->vmentry_ctrl        = _vmentry_control;
> +	vmcs_conf->misc	= misc_msr;
>  
>  	return 0;
>  }
> @@ -8241,11 +8245,9 @@ static __init int hardware_setup(void)
>  
>  	if (enable_preemption_timer) {
>  		u64 use_timer_freq = 5000ULL * 1000 * 1000;
> -		u64 vmx_msr;
>  
> -		rdmsrl(MSR_IA32_VMX_MISC, vmx_msr);
>  		cpu_preemption_timer_multi =
> -			vmx_msr & VMX_MISC_PREEMPTION_TIMER_RATE_MASK;
> +			vmcs_config.misc & VMX_MISC_PREEMPTION_TIMER_RATE_MASK;
>  
>  		if (tsc_khz)
>  			use_timer_freq = (u64)tsc_khz * 1000;
> -- 
> 2.35.3
>