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

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Message-ID: <2801b9d6-4f32-c4b2-ae93-c56ffc2b4621@intel.com>
Date:   Thu, 27 Jul 2023 15:10:31 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Chao Gao <chao.gao@...el.com>
CC:     <seanjc@...gle.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 1:16 PM, Chao Gao wrote:
>>>> +	case MSR_IA32_S_CET:
>>>> +	case MSR_KVM_GUEST_SSP:
>>>> +	case MSR_IA32_INT_SSP_TAB:
>>>> +		if (kvm_get_msr_common(vcpu, msr_info))
>>>> +			return 1;
>>>> +		if (msr_info->index == MSR_KVM_GUEST_SSP)
>>>> +			msr_info->data = vmcs_readl(GUEST_SSP);
>>>> +		else if (msr_info->index == MSR_IA32_S_CET)
>>>> +			msr_info->data = vmcs_readl(GUEST_S_CET);
>>>> +		else if (msr_info->index == MSR_IA32_INT_SSP_TAB)
>>>> +			msr_info->data = vmcs_readl(GUEST_INTR_SSP_TABLE);
>>>> +		break;
>>>> 	case MSR_IA32_DEBUGCTLMSR:
>>>> 		msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
>>>> 		break;
>>>> @@ -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.

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) )

^^^^^^^^^^^^^^^^^^^^^^^^^

+return 1;

>
>>>> +#define CET_LEG_BITMAP_BASE(data)	((data) >> 12)
>>>> +#define CET_EXCLUSIVE_BITS		(CET_SUPPRESS | CET_WAIT_ENDBR)
>>>> +	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
>>>> +		return kvm_set_msr_common(vcpu, msr_info);
>>> this hunk can be dropped as well.
>> In patch 16, these lines still need to be added back for PL{0,1,2}_SSP, so
>> would like keep it
> If that's the case, better to move it to patch 16, where the change
> can be justified. And PL3_SSP should be removed anyway. and then
> "msr_index != MSR_IA32_PL3_SSP" check in the below code snippet in
> patch 16 can go away.

Sure, will do it.

>
> +		/*
> +		 * Write to the base SSP MSRs should happen ahead of toggling
> +		 * of IA32_S_CET.SH_STK_EN bit.
> +		 */
> +		if (msr_index != MSR_IA32_PL3_SSP && data) {
> +			vmx_disable_write_intercept_sss_msr(vcpu);
> +			wrmsrl(msr_index, data);
> +		}
>
>
>> here.
>>
>>>> +		break;
>>>> +	case MSR_IA32_U_CET:
>>>> +	case MSR_IA32_S_CET:
>>>> +	case MSR_KVM_GUEST_SSP:
>>>> +	case MSR_IA32_INT_SSP_TAB:
>>>> +		if ((msr_index == MSR_IA32_U_CET ||
>>>> +		     msr_index == MSR_IA32_S_CET) &&
>>>> +		    ((data & ~VMX_CET_CONTROL_MASK) ||
>>>> +		     !IS_ALIGNED(CET_LEG_BITMAP_BASE(data), 4) ||
>>>> +		     (data & CET_EXCLUSIVE_BITS) == CET_EXCLUSIVE_BITS))
>>>> +			return 1;
>>> how about
>>>
>>> 	case MSR_IA32_U_CET:
>>> 	case MSR_IA32_S_CET:
>>> 		if ((data & ~VMX_CET_CONTROL_MASK) || ...
>>> 			...
>>>
>>> 	case MSR_KVM_GUEST_SSP:
>>> 	case MSR_IA32_INT_SSP_TAB:
>> Do you mean to use "fallthrough"?
> Yes.

OK, will change it, thanks!