Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH] KVM: VMX: Always treat MSR_IA32_PERF_CAPABILITIES as a
 valid PMU MSR
To:     Jim Mattson <jmattson@google.com>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        "Xu, Like" <like.xu@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Like Xu <like.xu@linux.intel.com>
References: <20200603203303.28545-1-sean.j.christopherson@intel.com>
 <46f57aa8-e278-b4fd-7ac8-523836308051@intel.com>
 <20200604151638.GD30223@linux.intel.com>
 <f7a234a7-664b-9160-f467-48b807d47c8b@redhat.com>
 <CALMp9eQoKJg36AabLErekJ-U10-DkRHW=dn14q0bxQHh7XrGMQ@mail.gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <285863c6-ef4b-a6f0-d223-ebc7cc515f84@redhat.com>
Date:   Thu, 4 Jun 2020 18:49:44 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.6.0
MIME-Version: 1.0
In-Reply-To: <CALMp9eQoKJg36AabLErekJ-U10-DkRHW=dn14q0bxQHh7XrGMQ@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 04/06/20 18:44, Jim Mattson wrote:
>>> I don't know if I would call it a "good assumption" so much as a "necessary
>>> assumption".  KVM_{GET,SET}_MSRS are allowed, and must function correctly,
>>> if they're called prior to KVM_SET_CPUID{2}.
>> Generally speaking this is not the case for the PMU; get_gp_pmc for
>> example depends on pmu->nr_arch_gp_counters which is initialized based
>> on CPUID leaf 0xA.
>>
>> The assumption that this patch fixes is that you can blindly take the
>> output of KVM_GET_MSR_INDEX_LIST and pass it to KVM_{GET,SET}_MSRS.
> 
> Is that an assumption or an invariant?

Both, I guess (a valid assumption for userspace, an invariant to be
respected for the kernel code).

The part where we don't fare to well, is that a bunch of MSRs that need
save/restore are _not_ included in KVM_GET_MSR_INDEX_LIST (and the PMU
is the biggest if not the only offender there).

Paolo