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

[Chao Gao]

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?

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.

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