linux-kernel - Re: [PATCH v3 5/9] KVM: vmx/pmu: Add MSR_ARCH_LBR

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Message-ID: <890a6f34-812a-5937-8761-d448a04f67d7@intel.com>
Date:   Thu, 4 Mar 2021 10:30:36 +0800
From:   "Xu, Like" <like.xu@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, wei.w.wang@...el.com,
        Borislav Petkov <bp@...en8.de>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH v3 5/9] KVM: vmx/pmu: Add MSR_ARCH_LBR_DEPTH emulation for
 Arch LBR

Hi Sean,

Thanks for your detailed review on the patch set.

On 2021/3/4 0:58, Sean Christopherson wrote:
> On Wed, Mar 03, 2021, Like Xu wrote:
>> @@ -348,10 +352,26 @@ static bool intel_pmu_handle_lbr_msrs_access(struct kvm_vcpu *vcpu,
>>   	return true;
>>   }
>>   
>> +/*
>> + * Check if the requested depth values is supported
>> + * based on the bits [0:7] of the guest cpuid.1c.eax.
>> + */
>> +static bool arch_lbr_depth_is_valid(struct kvm_vcpu *vcpu, u64 depth)
>> +{
>> +	struct kvm_cpuid_entry2 *best;
>> +
>> +	best = kvm_find_cpuid_entry(vcpu, 0x1c, 0);
>> +	if (best && depth && !(depth % 8))
> This is still wrong, it fails to weed out depth > 64.

How come ? The testcases depth = {65, 127, 128} get #GP as expected.

>
> Not that this is a hot path, but it's probably worth double checking that the
> compiler generates simple code for "depth % 8", e.g. it can be "depth & 7)".

Emm, the "%" operation is quite normal over kernel code.

if (best && depth && !(depth % 8))
    10659:       48 85 c0                test   rax,rax
    1065c:       74 c7                   je     10625 <intel_pmu_set_msr+0x65>
    1065e:       4d 85 e4                test   r12,r12
    10661:       74 c2                   je     10625 <intel_pmu_set_msr+0x65>
    10663:       41 f6 c4 07             test   r12b,0x7
    10667:       75 bc                   jne    10625 <intel_pmu_set_msr+0x65>

It looks like the compiler does the right thing.
Do you see the room for optimization ？

>
>> +		return (best->eax & 0xff) & (1ULL << (depth / 8 - 1));
>> +
>> +	return false;
>> +}
>> +