linux-kernel - Re: [PATCH 3/5] KVM: x86: Use MTRR macros to define possible MTRR MSR ranges

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Date:   Wed, 3 May 2023 23:23:59 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "guoke@...ontech.com" <guoke@...ontech.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "haiwenyao@...ontech.com" <haiwenyao@...ontech.com>
Subject: Re: [PATCH 3/5] KVM: x86: Use MTRR macros to define possible MTRR MSR
 ranges

On Wed, 2023-05-03 at 11:28 -0700, Sean Christopherson wrote:
> Use the MTRR macros to identify the ranges of possible MTRR MSRs instead
> of bounding the ranges with a mismash of open coded values and unrelated
> MSR indices.  Carving out the gap for the machine check MSRs in particular
> is confusing, as it's easy to incorrectly think the case statement handles
> MCE MSRs instead of skipping them.
> 
> Drop the range-based funneling of MSRs between the end of the MCE MSRs
> and MTRR_DEF_TYPE, i.e. 0x2A0-0x2FF, and instead handle MTTR_DEF_TYPE as
> the one-off case that it is.
> 
> Extract PAT (0x277) as well in anticipation of dropping PAT "handling"
> from the MTRR code.
> 
> Keep the range-based handling for the variable+fixed MTRRs even though
> capturing unknown MSRs 0x214-0x24F is arguably "wrong".  There is a gap in
> the fixed MTRRs, 0x260-0x267, i.e. the MTRR code needs to filter out
> unknown MSRs anyways, 
> 

Looks a little bit half measure, but ...

> and using a single range generates marginally better
> code for the big switch statement.

could you educate why because I am ignorant of compiler behaviour? :)

> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/mtrr.c |  2 +-
>  arch/x86/kvm/x86.c  | 10 ++++++----
>  2 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c
> index 9fac1ec03463..d2c428f4ae42 100644
> --- a/arch/x86/kvm/mtrr.c
> +++ b/arch/x86/kvm/mtrr.c
> @@ -28,7 +28,7 @@
>  static bool msr_mtrr_valid(unsigned msr)
>  {
>  	switch (msr) {
> -	case 0x200 ... 0x200 + 2 * KVM_NR_VAR_MTRR - 1:
> +	case MTRRphysBase_MSR(0) ... MTRRphysMask_MSR(KVM_NR_VAR_MTRR - 1):
>  	case MSR_MTRRfix64K_00000:
>  	case MSR_MTRRfix16K_80000:
>  	case MSR_MTRRfix16K_A0000:
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e7f78fe79b32..8b356c9d8a81 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3700,8 +3700,9 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  			return 1;
>  		}
>  		break;
> -	case 0x200 ... MSR_IA32_MC0_CTL2 - 1:
> -	case MSR_IA32_MCx_CTL2(KVM_MAX_MCE_BANKS) ... 0x2ff:
> +	case MSR_IA32_CR_PAT:
> +	case MTRRphysBase_MSR(0) ... MSR_MTRRfix4K_F8000:
> +	case MSR_MTRRdefType:
>  		return kvm_mtrr_set_msr(vcpu, msr, data);
>  	case MSR_IA32_APICBASE:
>  		return kvm_set_apic_base(vcpu, msr_info);
> @@ -4108,9 +4109,10 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		msr_info->data = kvm_scale_tsc(rdtsc(), ratio) + offset;
>  		break;
>  	}
> +	case MSR_IA32_CR_PAT:
>  	case MSR_MTRRcap:
> -	case 0x200 ... MSR_IA32_MC0_CTL2 - 1:
> -	case MSR_IA32_MCx_CTL2(KVM_MAX_MCE_BANKS) ... 0x2ff:
> +	case MTRRphysBase_MSR(0) ... MSR_MTRRfix4K_F8000:
> +	case MSR_MTRRdefType:
>  		return kvm_mtrr_get_msr(vcpu, msr_info->index, &msr_info->data);
>  	case 0xcd: /* fsb frequency */
>  		msr_info->data = 3;