linux-kernel - Re: [PATCH v4 13/20] KVM:VMX: Emulate read and write to CET MSRs

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Message-ID: <10511c2d-7b64-374d-cd91-ddac2c1a788e@intel.com>
Date:   Fri, 28 Jul 2023 08:43:10 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     Chao Gao <chao.gao@...el.com>, <pbonzini@...hat.com>,
        <peterz@...radead.org>, <john.allen@....com>,
        <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <rick.p.edgecombe@...el.com>, <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH v4 13/20] KVM:VMX: Emulate read and write to CET MSRs


On 7/27/2023 11:20 PM, Sean Christopherson wrote:
> On Thu, Jul 27, 2023, Weijiang Yang wrote:
>> On 7/27/2023 1:16 PM, Chao Gao wrote:
>>>>>> @@ -2402,6 +2417,31 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>>>>> 		else
>>>>>> 			vmx->pt_desc.guest.addr_a[index / 2] = data;
>>>>>> 		break;
>>>>>> +#define VMX_CET_CONTROL_MASK		(~GENMASK_ULL(9, 6))
>>>>> bits9-6 are reserved for both intel and amd. Shouldn't this check be
>>>>> done in the common code?
>>>> My thinking is, on AMD platform, bit 63:2 is anyway reserved since it doesn't
>>>> support IBT,
>>> You can only say
>>>
>>> 	bits 5:2 and bits 63:10 are reserved since AMD doens't support IBT.
>>>
>>> bits 9:6 are reserved regardless of the support of IBT.
>>>
>>>> so the checks in common code for AMD is enough, when the execution flow comes
>>>> here,
>>>>
>>>> it should be vmx, and need this additional check.
>>> The checks against reserved bits are common for AMD and Intel:
>>>
>>> 1. if SHSTK is supported, bit1:0 are not reserved.
>>> 2. if IBT is supported, bit5:2 and bit63:10 are not reserved
>>> 3. bit9:6 are always reserved.
>>>
>>> There is nothing specific to Intel.
> +1
>
>> So you want the code to be:
>>
>> +#define CET_IBT_MASK_BITS          (GENMASK_ULL(5, 2) | GENMASK_ULL(63,
>> 10))
>>
>> +#define CET_CTRL_RESERVED_BITS GENMASK(9, 6)
>>
>> +#define CET_SHSTK_MASK_BITSGENMASK(1, 0)
>>
>> +if ((!guest_can_use(vcpu, X86_FEATURE_SHSTK) &&
>>
>> +(data & CET_SHSTK_MASK_BITS)) ||
>>
>> +(!guest_can_use(vcpu, X86_FEATURE_IBT) &&
>>
>> +(data & CET_IBT_MASK_BITS)) ||
>>
>>                              (data & CET_CTRL_RESERVED_BITS) )
>>
>> ^^^^^^^^^^^^^^^^^^^^^^^^^
> Yes, though I vote to separate each check, e.g.
>
> 	if (data & CET_CTRL_RESERVED_BITS)
> 		return 1;
>
> 	if (!guest_can_use(vcpu, X86_FEATURE_SHSTK) && (data & CET_SHSTK_MASK_BITS))
> 		return 1;
>
> 	if (!guest_can_use(vcpu, X86_FEATURE_IBT) && (data & CET_IBT_MASK_BITS))
> 		return 1;
>
> I would expect the code generation to be similar, if not outright identical, and
> IMO it's easier to quickly understand the flow if each check is a separate if-statement.

It looks good to me! Thank you!