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Subject: Re: [PATCH v4 2/5] KVM: x86: Add IBPB support
To:     KarimAllah Ahmed <karahmed@amazon.com>,
        Jim Mattson <jmattson@google.com>,
        KarimAllah Ahmed <karahmed@amazon.de>
Cc:     the arch/x86 maintainers <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        David Woodhouse <dwmw@amazon.co.uk>
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <200c5e32-e238-ee87-45ed-4701285ff489@redhat.com>
Date:   Wed, 31 Jan 2018 12:15:35 -0500
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On 31/01/2018 12:11, KarimAllah Ahmed wrote:
> On 01/31/2018 05:50 PM, Jim Mattson wrote:
>> On Wed, Jan 31, 2018 at 5:10 AM, KarimAllah Ahmed <karahmed@amazon.de>
>> wrote:
>>
>>> +               vmx_disable_intercept_for_msr(vmx->vmcs01.msr_bitmap,
>>> MSR_IA32_PRED_CMD,
>>> +                                             MSR_TYPE_W);
>>
>> Why not disable this intercept eagerly, rather than lazily? Unlike
>> MSR_IA32_SPEC_CTRL, there is no guest value to save/restore, so there
>> is no cost to disabling the intercept if the guest cpuid info declares
>> support for it.
>>
>>
>>> +       if (to_vmx(vcpu)->save_spec_ctrl_on_exit) {
>>> +               nested_vmx_disable_intercept_for_msr(
>>> +                               msr_bitmap_l1, msr_bitmap_l0,
>>> +                               MSR_IA32_PRED_CMD,
>>> +                               MSR_TYPE_R);
>>> +       }
>>
>> I don't think this should be predicated on
>> "to_vmx(vcpu)->save_spec_ctrl_on_exit." Why not just
>> "guest_cpuid_has(vcpu, X86_FEATURE_IBPB)"?
> 
> Paolo suggested this on the previous revision because guest_cpuid_has()
> would be slow.

Sorry, that was for spec_ctrl.  Here there's no need to do any kind of
conditional check.

Paolo

>> Also, the final argument to
>> nested_vmx_disable_intercept_for_msr should be MSR_TYPE_W rather than
>> MSR_TYPE_R.
>>
> Oops! will fix!
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