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Date:   Tue, 25 Apr 2023 10:35:25 +0800
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Subject: Re: [PATCH 1/6] KVM: x86: Virtualize CR4.LASS
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To:     "Gao, Chao" <chao.gao@intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        "Christopherson,, Sean" <seanjc@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        H Peter Anvin <hpa@zytor.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20230420133724.11398-1-guang.zeng@intel.com>
 <20230420133724.11398-2-guang.zeng@intel.com> <ZEYwlMmzYnJjNNHq@chao-email>
From:   Zeng Guang <guang.zeng@intel.com>
In-Reply-To: <ZEYwlMmzYnJjNNHq@chao-email>
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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.