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

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Message-ID: <848cb820-e634-4608-5d09-a9bd9ee6de18@intel.com>
Date:   Tue, 25 Apr 2023 10:35:25 +0800
From:   Zeng Guang <guang.zeng@...el.com>
To:     "Gao, Chao" <chao.gao@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "Christopherson,, Sean" <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" <kvm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] KVM: x86: Virtualize CR4.LASS


On 4/24/2023 3:32 PM, Gao, Chao wrote:
> 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.

Essentially it allows nested guest to set CR4.LASS. L1 guest uses 
cr4_fixed1 to check
cr4 value requested to set by nested guest valid or not. Nested guest 
will get #GP
fault if it's not allowed.

> 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.