linux-kernel - Re: [RFC PATCH v2 04/11] KVM: VMX: Add IA32_SPEC

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Date:   Tue, 16 May 2023 15:16:59 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Chao Gao <chao.gao@...el.com>, kvm@...r.kernel.org
Cc:     Jiaan Lu <jiaan.lu@...el.com>, Zhang Chen <chen.zhang@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 04/11] KVM: VMX: Add IA32_SPEC_CTRL virtualization
 support

On 4/14/2023 2:25 PM, Chao Gao wrote:

...

> +static inline void vmx_set_guest_spec_ctrl(struct vcpu_vmx *vmx, u64 val)
> +{
> +	vmx->guest_spec_ctrl = val;
> +
> +	/*
> +	 * For simplicity, always keep IA32_SPEC_CTRL_SHADOW up-to-date,
> +	 * regardless of the MSR intercept state.
> +	 */
> +	if (cpu_has_spec_ctrl_virt())
> +		vmcs_write64(IA32_SPEC_CTRL_SHADOW, val);
> +
> +	/*
> +	 * Update the effective value of IA32_SPEC_CTRL to reflect changes to
> +	 * guest's IA32_SPEC_CTRL. Bits in the mask should always be set.
> +	 */

Why bits in the mask should always be set?

The bits set in the mask only means them cannot be modified by guest. 
KVM can use the mask to force the bits to 0 as well.

> +	vmx->spec_ctrl = val | vmx_get_spec_ctrl_mask(vmx);
> +}
>   #endif /* __KVM_X86_VMX_H */