linux-kernel - Re: [PATCH 1/6] KVM: x86: Virtualize CR4.LASS

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Message-ID: <ZEYwlMmzYnJjNNHq@chao-email>
Date:   Mon, 24 Apr 2023 15:32:36 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Zeng Guang <guang.zeng@...el.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        H Peter Anvin <hpa@...or.com>, <kvm@...r.kernel.org>,
        <x86@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] KVM: x86: Virtualize CR4.LASS

On Thu, Apr 20, 2023 at 09:37:19PM +0800, Zeng Guang wrote:
>Virtualize CR4.LASS[bit 27] under KVM control instead of being guest-owned
>as CR4.LASS generally set once for each vCPU at boot time and won't be
>toggled at runtime. Besides, only if VM has LASS capability enumerated with
>CPUID.(EAX=07H.ECX=1):EAX.LASS[bit 6], KVM allows guest software to be able
>to set CR4.LASS.

>By design CR4.LASS can be manipulated by nested guest as
>well.

This is inaccurate. The change in nested_vmx_cr_fixed1_bits_update() is
to allow L1 guests to set CR4.LASS in VMX operation. I would say:

Set the CR4.LASS bit in the emulated IA32_VMX_CR4_FIXED1 MSR for guests
to allow guests to enable LASS in nested VMX operation.