linux-kernel - Re: [RFC PATCH 15/41] KVM: x86/pmu: Manage MSR interception for IA32_PERF_GLOBAL

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Date: Thu, 11 Apr 2024 14:21:40 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xiong Zhang <xiong.y.zhang@...ux.intel.com>
Cc: pbonzini@...hat.com, peterz@...radead.org, mizhang@...gle.com, 
	kan.liang@...el.com, zhenyuw@...ux.intel.com, dapeng1.mi@...ux.intel.com, 
	jmattson@...gle.com, kvm@...r.kernel.org, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org, zhiyuan.lv@...el.com, eranian@...gle.com, 
	irogers@...gle.com, samantha.alt@...el.com, like.xu.linux@...il.com, 
	chao.gao@...el.com, Xiong Zhang <xiong.y.zhang@...el.com>
Subject: Re: [RFC PATCH 15/41] KVM: x86/pmu: Manage MSR interception for IA32_PERF_GLOBAL_CTRL

On Fri, Jan 26, 2024, Xiong Zhang wrote:
> +	if (is_passthrough_pmu_enabled(&vmx->vcpu)) {
> +		/*
> +		 * Setup auto restore guest PERF_GLOBAL_CTRL MSR at vm entry.
> +		 */
> +		if (vmentry_ctrl & VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL)
> +			vmcs_write64(GUEST_IA32_PERF_GLOBAL_CTRL, 0);
> +		else {
> +			i = vmx_find_loadstore_msr_slot(&vmx->msr_autoload.guest,
> +						       MSR_CORE_PERF_GLOBAL_CTRL);
> +			if (i < 0) {
> +				i = vmx->msr_autoload.guest.nr++;
> +				vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT,
> +					     vmx->msr_autoload.guest.nr);
> +			}
> +			vmx->msr_autoload.guest.val[i].index = MSR_CORE_PERF_GLOBAL_CTRL;
> +			vmx->msr_autoload.guest.val[i].value = 0;

Eww, no.   Just make cpu_has_load_perf_global_ctrl() and VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL
hard requirements for enabling passthrough mode.  And then have clear_atomic_switch_msr()
yell if KVM tries to disable loading MSR_CORE_PERF_GLOBAL_CTRL.