linux-kernel - Re: [PATCH v5 15/19] KVM:x86: Optimize CET supervisor SSP save/reload

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Date:   Fri, 4 Aug 2023 16:43:12 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Yang Weijiang <weijiang.yang@...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 v5 15/19] KVM:x86: Optimize CET supervisor SSP save/reload

On Thu, Aug 03, 2023 at 12:27:28AM -0400, Yang Weijiang wrote:
>Make PL{0,1,2}_SSP as write-intercepted to detect whether
>guest is using these MSRs. Disable intercept to the MSRs
>if they're written with non-zero values. KVM does save/
>reload for the MSRs only if they're used by guest.

What would happen if guest tries to use XRSTORS to load S_CET state from a
xsave area without any writes to the PL0-2_SSP (i.e., at that point, writes to
the MSRs are still intercepted)?

>@@ -2420,6 +2432,14 @@ 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;
>+	case MSR_IA32_PL0_SSP ... MSR_IA32_PL2_SSP:
>+		if (kvm_set_msr_common(vcpu, msr_info))
>+			return 1;
>+		if (data) {
>+			vmx_disable_write_intercept_sss_msr(vcpu);
>+			wrmsrl(msr_index, data);

Is it necessary to do the wrmsl()?
looks the next kvm_x86_prepare_switch_to_guest() will load PL0-2_SSP from the
caching values.