linux-kernel - Re: [PATCH 3/3] KVM: VMX: simplify MSR interception enable/disable

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Date: Wed, 21 Feb 2024 08:50:04 -0800
From: Dongli Zhang <dongli.zhang@...cle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, pbonzini@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] KVM: VMX: simplify MSR interception enable/disable



On 2/21/24 07:43, Sean Christopherson wrote:
> On Tue, Feb 20, 2024, Dongli Zhang wrote:
>> Hi Sean,
>>
>> On 2/19/24 14:33, Sean Christopherson wrote:
>>> On Fri, Feb 16, 2024, Dongli Zhang wrote:
>>>> ---
>>>>  arch/x86/kvm/vmx/vmx.c | 55 +++++++++++++++++++++---------------------
>>>>  1 file changed, 28 insertions(+), 27 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>>>> index 5a866d3c2bc8..76dff0e7d8bd 100644
>>>> --- a/arch/x86/kvm/vmx/vmx.c
>>>> +++ b/arch/x86/kvm/vmx/vmx.c
>>>> @@ -669,14 +669,18 @@ static int possible_passthrough_msr_slot(u32 msr)
>>>>  	return -ENOENT;
>>>>  }
>>>>  
>>>> -static bool is_valid_passthrough_msr(u32 msr)
>>>> +#define VMX_POSSIBLE_PASSTHROUGH	1
>>>> +#define VMX_OTHER_PASSTHROUGH		2
>>>> +/*
>>>> + * Vefify if the msr is the passthrough MSRs.
>>>> + * Return the index in *possible_idx if it is a possible passthrough MSR.
>>>> + */
>>>> +static int validate_passthrough_msr(u32 msr, int *possible_idx)
>>>
>>> There's no need for a custom tri-state return value or an out-param, just return
>>> the slot/-ENOENT.  Not fully tested yet, but this should do the trick.
>>
>> The new patch looks good to me, from functionality's perspective.
>>
>> Just that the new patched function looks confusing. That's why I was adding the
>> out-param initially to differentiate from different cases.
>>
>> The new vmx_get_passthrough_msr_slot() is just doing the trick by combining many
>> jobs together:
>>
>> 1. Get the possible passthrough msr slot index.
>>
>> 2. For x2APIC/PT/LBR msr, return -ENOENT.
>>
>> 3. For other msr, return the same -ENOENT, with a WARN.
>>
>> The semantics of the function look confusing.
>>
>> If the objective is to return passthrough msr slot, why return -ENOENT for
>> x2APIC/PT/LBR.
> 
> Because there is no "slot" for them in vmx_possible_passthrough_msrs, and the
> main purpose of the helpers is to get that slot in order to efficiently update
> the MSR bitmaps in response to userspace MSR filter changes.  The WARN is an extra
> sanity check to ensure that KVM doesn't start passing through an MSR without
> adding the MSR to vmx_possible_passthrough_msrs (or special casing it a la XAPIC,
> PT, and LBR MSRS).
> 
>> Why both x2APIC/PT/LBR and other MSRs return the same -ENOENT, while the other
>> MSRs may trigger WARN. (I know this is because the other MSRs do not belong to
>> any passthrough MSRs).
> 
> The x2APIC/PT/LBR MSRs are given special treatment: KVM may pass them through to
> the guest, but unlike the "regular" passthrough MSRs, userspace is NOT allowed to
> override that behavior via MSR filters.
> 
> And so as mentioned above, they don't have a slot in vmx_possible_passthrough_msrs.

Thank you very much for the explanation! This looks good to me.

Dongli Zhang