linux-kernel - Re: [PATCH v4 16/20] KVM:x86: Optimize CET supervisor SSP save/reload

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Message-ID: <74a728c0-6f63-260b-de9d-348fb6efb41b@intel.com>
Date:   Thu, 27 Jul 2023 14:23:51 +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 16/20] KVM:x86: Optimize CET supervisor SSP save/reload


On 7/27/2023 11:27 AM, Chao Gao wrote:
> On Thu, Jul 20, 2023 at 11:03:48PM -0400, Yang Weijiang wrote:
>> /*
>>   * Writes msr value into the appropriate "register".
>>   * Returns 0 on success, non-0 otherwise.
>> @@ -2427,7 +2439,16 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>> #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);
>> +		if (kvm_set_msr_common(vcpu, msr_info))
>> +			return 1;
>> +		/*
>> +		 * Write to the base SSP MSRs should happen ahead of toggling
>> +		 * of IA32_S_CET.SH_STK_EN bit.
> Is this a requirement from SDM? And how is this related to the change below?

No, after a second thought, the usage of the supervisor SSPs doesn't 
necessary mean

supervisor SHSTK is being enabled, e.g., used as some HW registers. I'll 
remove it.

>
> Note that PLx_SSP MSRs are linear addresses of shadow stacks for different CPLs.
> I may think using the page at 0 (assuming 0 is the reset value of PLx SSP) is
> allowed in architecture although probably no kernel will do so.
>
> I don't understand why this comment is needed. I suggest dropping it.

will drop it, thanks!

>
>> +		 */
>> +		if (msr_index != MSR_IA32_PL3_SSP && data) {
>> +			vmx_disable_write_intercept_sss_msr(vcpu);
>> +			wrmsrl(msr_index, data);
>> +		}