Received-SPF: pass (google.com: domain of linux-kernel+bounces-176365-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Date: Sat, 11 May 2024 09:53:47 +0800
From: Chao Gao <chao.gao@intel.com>
To: "Li, Xin3" <xin3.li@intel.com>
CC: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>, "linux-doc@vger.kernel.org"
	<linux-doc@vger.kernel.org>, "linux-kselftest@vger.kernel.org"
	<linux-kselftest@vger.kernel.org>, "seanjc@google.com" <seanjc@google.com>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>, "corbet@lwn.net"
	<corbet@lwn.net>, "tglx@linutronix.de" <tglx@linutronix.de>,
	"mingo@redhat.com" <mingo@redhat.com>, "bp@alien8.de" <bp@alien8.de>,
	"dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>, "x86@kernel.org"
	<x86@kernel.org>, "hpa@zytor.com" <hpa@zytor.com>, "shuah@kernel.org"
	<shuah@kernel.org>, "vkuznets@redhat.com" <vkuznets@redhat.com>,
	"peterz@infradead.org" <peterz@infradead.org>, "Shankar, Ravi V"
	<ravi.v.shankar@intel.com>, "xin@zytor.com" <xin@zytor.com>
Subject: Re: [PATCH v2 11/25] KVM: x86: Add kvm_is_fred_enabled()
Message-ID: <Zj7Pq2fnU+YcNu0x@chao-email>
References: <20240207172646.3981-1-xin3.li@intel.com>
 <20240207172646.3981-12-xin3.li@intel.com>
 <Zi9ZSuwQXl8/Ncs/@chao-email>
 <SA1PR11MB6734D9EF8C2D9A762CBEBF07A8E02@SA1PR11MB6734.namprd11.prod.outlook.com>
Content-Type: text/plain; charset="us-ascii"
Content-Disposition: inline
In-Reply-To: <SA1PR11MB6734D9EF8C2D9A762CBEBF07A8E02@SA1PR11MB6734.namprd11.prod.outlook.com>
Precedence: bulk
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?9vLJh8wakBp7RN38sDT2DrbdDacjxL2UCmznyu2huLxh0jO5a5qHFyVfJrvu?=
 =?us-ascii?Q?bf8fBL/gUA/ufaB/urXon3GZx1YbkPb/IZ0dl4ob4sdUNgBU28g3V7raf29u?=
 =?us-ascii?Q?F+xM0h7ECXfprNs4XyINsBkyAH5ChdJmzGQ3tkZNiQXxnztRHvfmB13coW0q?=
 =?us-ascii?Q?9r8P/JZchk6wgiPdDwFGF5cNJWXZiDVmrLEFpFxaNFED2X6uGbjK20KFTemy?=
 =?us-ascii?Q?4bAKh/KrNPSEoWesQP53TOzfGbqfV81EeqKmEAiV34fOwKoJxDZE5uud2ZJX?=
 =?us-ascii?Q?rqGSJpD8pMW6cSQg5Xs09/57KUer/07xcOU5KMCYXASRx72TW9QX4CCCR8Ne?=
 =?us-ascii?Q?vixCbj3v/nux+KsKo5A+lMjTHOPFJgNawhbQzw3rTEGA1XBrO1W8/zH7wdg1?=
 =?us-ascii?Q?gcwuVm9tz3WVfhvU0xLwyGVKzwd3q0a55tBxYfYhczMzN2tBHm+9Z+pq4yxl?=
 =?us-ascii?Q?ofWSus0naYyYph7EGJDWN7szFTJW4Gy9q53HqFJdCIa5LgKTNaq/9mh0cy7p?=
 =?us-ascii?Q?01rRUVjT0FlslDQ2VwonnHY4nfKoEdZ2Mt4IIlG3tpPXp7RMS5CzXAwbMnL1?=
 =?us-ascii?Q?XEiwYtYw3aLlJ3Ye1Mbi+Gw+srYPE7cmEOIaPT+RigT1Q7PZTOAUsUrma7A4?=
 =?us-ascii?Q?WYV9bqzosxTHM+Thg6+h9EJvCjA6/q33O/tt24EbUtyAZhODXakMrmV1HkgV?=
 =?us-ascii?Q?tsZ1UDasyR6cXNozfG26Kg+NZ4qvrPiMZ8RtIALtlYdWtVQKOLd++k1U2jJL?=
 =?us-ascii?Q?mXx9g7OoyVDL4qApuLYC/RS3hyl+rI83eiAqMv2gelATbu5Qlluyr/qx7ax4?=
 =?us-ascii?Q?RZCrlofPzebZBa0ckWZFAevTW9ioL7rJhvkaA1xd5CeX5o3Ql20W6VTUCw/G?=
 =?us-ascii?Q?/odC9EdvWilOezyKzomQjJGqi1qQqMzv8GiQ5CRpo4/Y5SvFzntdVNBDd/fF?=
 =?us-ascii?Q?xrGQCLaa5zdHhzOLmFD7HK5asdJp67/C/4mUgJEdLJQKD9zfyObXcpDSmYC5?=
 =?us-ascii?Q?BOZEOGBBnM8VfV2VwlDd9p846NiMlKa6sN28l+B1JQG0Uwmf+QZs18WMM8dl?=
 =?us-ascii?Q?pI5qUUlh3axjswlRvmUXPc0rj/Bn9Oz9EaRhNkD+uOQrQglJXUhLj+FByKzN?=
 =?us-ascii?Q?Vb0RDzPOKSCVpJOTxuMC0/xwbJXlrfgA3YFog7QDdVCRp2IHoKJkzIvzc6I8?=
 =?us-ascii?Q?/6pL3u0aN6CxGgK6PAqZ46Nj0AratSB0NN78NKQ7xBeZus7HmQ7AGWxs53jI?=
 =?us-ascii?Q?DvYaJhaz44SeBqeJkW8kTe3lcduYCQ5sUl/Ui+0ZXYcqUHEM2X64Mse3qEl7?=
 =?us-ascii?Q?vHLtg9FNYOCzyhc+8PV0rb9VaGDoF2yAfX6QRbUuE4i06OrtD8kBs9j/lWRK?=
 =?us-ascii?Q?5ORtwW08funRL5H1vZlXGzyIxTTVSWdri+XNQYLJ2u/mQuRZczjwIULn0F5n?=
 =?us-ascii?Q?micBz7oRSi89qpjxYhEJ8QPBsTjvyESfezAtWgigW7yyzL5GI0sr0O4Nzm0D?=
 =?us-ascii?Q?WshIuLF9gj+GDU4/s1ZgFt3YuLt0qNQv7Tv/0+nLIN5kpBpMu/AwIIp4K6L5?=
 =?us-ascii?Q?oScIW22BE9LVzXCLfgLU0L8UT1MgiaSVlIJW7pSE?=
X-MS-Exchange-CrossTenant-Network-Message-Id: 65f3a90d-1202-4114-7a4f-08dc715d3964
X-MS-Exchange-CrossTenant-AuthSource: CH3PR11MB8660.namprd11.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 11 May 2024 01:53:59.0326
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 46c98d88-e344-4ed4-8496-4ed7712e255d
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: VqnPq9f+WRd2mc1LHm2aAthbEg+fauJYVqJ5XSDRFjh1cxaCItVJPDebySSJJsX8OiVfMpKjtfAukNHAe5b9Dg==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH7PR11MB7075
X-OriginatorOrg: intel.com

On Sat, May 11, 2024 at 09:24:12AM +0800, Li, Xin3 wrote:
>> >+/*
>> >+ * It's enough to check just CR4.FRED (X86_CR4_FRED) to tell if
>> >+ * a vCPU is running with FRED enabled, because:
>> >+ * 1) CR4.FRED can be set to 1 only _after_ IA32_EFER.LMA = 1.
>> >+ * 2) To leave IA-32e mode, CR4.FRED must be cleared first.
>> >+ *
>> >+ * More details at FRED Spec 6.0 Section 4.2 Enabling in CR4.
>> >+ */
>> 
>> I think we can give more context here, e.g.,
>> 
>> Although FRED architecture applies to 64-bit mode only, there is no need to check if
>> the CPU is in 64-bit mode (i.e., IA32_EFER.LMA and CS.L) to tell if FRED is enabled
>> because CR4.FRED=1 implies the CPU is in 64-bit mode.
>
>What is "more context" here?

e.g.,
why IA32_EFER.LMA and CPU mode are related to FRED here?

"it's enough to " implies something else is not necessary. what is it?

I don't think the original comment make them super clear.

>
>> Specifically,
>
>