linux-kernel - Re: [PATCH] KVM: x86: VMX: hyper-v: Enlightened MSR-Bitmap support

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Message-ID: <87muy7423a.fsf@vitty.brq.redhat.com>
Date:   Fri, 13 Apr 2018 16:23:37 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: VMX: hyper-v: Enlightened MSR-Bitmap support

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 12/04/2018 17:25, Vitaly Kuznetsov wrote:
>> @@ -5335,6 +5353,9 @@ static void __always_inline vmx_disable_intercept_for_msr(unsigned long *msr_bit
>>  	if (!cpu_has_vmx_msr_bitmap())
>>  		return;
>>  
>> +	if (static_branch_unlikely(&enable_emsr_bitmap))
>> +		evmcs_touch_msr_bitmap();
>> +
>>  	/*
>>  	 * See Intel PRM Vol. 3, 20.6.9 (MSR-Bitmap Address). Early manuals
>>  	 * have the write-low and read-high bitmap offsets the wrong way round.
>> @@ -5370,6 +5391,9 @@ static void __always_inline vmx_enable_intercept_for_msr(unsigned long *msr_bitm
>>  	if (!cpu_has_vmx_msr_bitmap())
>>  		return;
>>  
>> +	if (static_branch_unlikely(&enable_emsr_bitmap))
>> +		evmcs_touch_msr_bitmap();
>
> I'm not sure about the "unlikely".  Can you just check current_evmcs
> instead (dropping the static key completely)?

current_evmcs is just a cast:

 (struct hv_enlightened_vmcs *)this_cpu_read(current_vmcs)

so it is always not NULL here :-) We need to check enable_evmcs static
key first. Getting rid of the newly added enable_emsr_bitmap is, of
course, possible.

(Actually, we only call vmx_{dis,en}able_intercept_for_msr in the very
beginning of vCPUs life so this is not a hotpath and likeliness doesn't
really matter).

Will do v2 without the static key, thanks!

>
> The function, also, is small enough that inlining should be beneficial.
>
> Paolo

-- 
  Vitaly