linux-kernel - Re: [PATCH v2 17/25] KVM: nVMX: Add support for the secondary VM exit controls

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Date: Thu, 13 Jun 2024 11:11:41 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xin Li <xin3.li@...el.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	pbonzini@...hat.com, corbet@....net, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	shuah@...nel.org, vkuznets@...hat.com, peterz@...radead.org, 
	ravi.v.shankar@...el.com, xin@...or.com
Subject: Re: [PATCH v2 17/25] KVM: nVMX: Add support for the secondary VM exit controls

On Wed, Feb 07, 2024, Xin Li wrote:
> @@ -6856,13 +6865,17 @@ static void nested_vmx_setup_exit_ctls(struct vmcs_config *vmcs_conf,
>  		VM_EXIT_HOST_ADDR_SPACE_SIZE |
>  #endif
>  		VM_EXIT_LOAD_IA32_PAT | VM_EXIT_SAVE_IA32_PAT |
> -		VM_EXIT_CLEAR_BNDCFGS;
> +		VM_EXIT_CLEAR_BNDCFGS | VM_EXIT_ACTIVATE_SECONDARY_CONTROLS;
>  	msrs->exit_ctls_high |=
>  		VM_EXIT_ALWAYSON_WITHOUT_TRUE_MSR |
>  		VM_EXIT_LOAD_IA32_EFER | VM_EXIT_SAVE_IA32_EFER |
>  		VM_EXIT_SAVE_VMX_PREEMPTION_TIMER | VM_EXIT_ACK_INTR_ON_EXIT |
>  		VM_EXIT_LOAD_IA32_PERF_GLOBAL_CTRL;
>  
> +	/* secondary exit controls */

Drop the comment, it's pretty obvious what field is being setup.

> +	if (msrs->exit_ctls_high & VM_EXIT_ACTIVATE_SECONDARY_CONTROLS)
> +		rdmsrl(MSR_IA32_VMX_EXIT_CTLS2, msrs->secondary_exit_ctls);

This is wrong, the resulting msrs->secondary_exit_ctls needs to be sanitized
based on what KVM supports for nVMX.

On a very related topic, this should not do a raw RDMSR.  One of the reasons why
KVM uses vmcs_config as the base is to avoid advertising features to L1 that KVM
itself doesn't support, e.g. because the expected entry+exit pairs aren't
supported.

And by pulling state from vmcs_conf->secondary_exit_ctls there's no need to check
the activation bit.

I.e. literaly just this:

	msrs->secondary_exit_ctls = vmcs_conf->secondary_exit_ctls;
	msrs->secondary_exit_ctls &= 0;

and then when nVMX FRED support is ready, it becomes:

	msrs->secondary_exit_ctls = vmcs_conf->secondary_exit_ctls;
	msrs->secondary_exit_ctls &= SECONDARY_VM_EXIT_SAVE_IA32_FRED |
				     SECONDARY_VM_EXIT_LOAD_IA32_FRED;