linux-kernel - Re: [RFC PATCH 28/41] KVM: x86/pmu: Switch IA32_PERF_GLOBAL

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Date: Thu, 11 Apr 2024 14:54:09 -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 28/41] KVM: x86/pmu: Switch IA32_PERF_GLOBAL_CTRL at
 VM boundary

On Fri, Jan 26, 2024, Xiong Zhang wrote:
> +static void save_perf_global_ctrl_in_passthrough_pmu(struct vcpu_vmx *vmx)
> +{
> +	struct kvm_pmu *pmu = vcpu_to_pmu(&vmx->vcpu);
> +	int i;
> +
> +	if (vm_exit_controls_get(vmx) & VM_EXIT_SAVE_IA32_PERF_GLOBAL_CTRL) {
> +		pmu->global_ctrl = vmcs_read64(GUEST_IA32_PERF_GLOBAL_CTRL);
> +	} else {
> +		i = vmx_find_loadstore_msr_slot(&vmx->msr_autostore.guest,
> +						MSR_CORE_PERF_GLOBAL_CTRL);
> +		if (i < 0)
> +			return;
> +		pmu->global_ctrl = vmx->msr_autostore.guest.val[i].value;

As before, NAK to using the MSR load/store lists unless there's a *really* good
reason I'm missing.

And we should consider adding VCPU_EXREG_GLOBAL_CTRL so that we can defer the
VMREAD until KVM actually tries to access the guest value.