Received-SPF: pass (google.com: domain of linux-kernel+bounces-161904-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Date: Mon, 29 Apr 2024 16:24:42 +0800
From: Chao Gao <chao.gao@intel.com>
To: Xin Li <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: <Zi9ZSuwQXl8/Ncs/@chao-email>
References: <20240207172646.3981-1-xin3.li@intel.com>
 <20240207172646.3981-12-xin3.li@intel.com>
Content-Type: text/plain; charset="us-ascii"
Content-Disposition: inline
In-Reply-To: <20240207172646.3981-12-xin3.li@intel.com>
Precedence: bulk
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?rIlJQ2kdfe1JKbZEtszZheabW/RLeJRdwmh3dcA3b9djtWBXY39wGz8XHhWK?=
 =?us-ascii?Q?vQMpvpCXJp+BdIWbB8GviRTpj2HjFCX70685DHiJ6VtSOqpe1rZZHmmWujoU?=
 =?us-ascii?Q?d8HxhLsO+cog71Lpw82f6GFYDtJPZTMHkZYbLyJ2CqLOyf30yvUeg6Q3H7P8?=
 =?us-ascii?Q?fpsF26kIBS8JpGO8MWk8DkCyoo6Bg4T6C7EthtVFrrdTmYm7cEUsKL2+56yM?=
 =?us-ascii?Q?L7/rsub9mA49pUy7xMj3I+ZnXQYlDsjQjtE7CtRzjWT3qHkPM88nbxf4FRyu?=
 =?us-ascii?Q?XYmfLQ8EuloqOqVh5NkKDi4tXAAXpkO4KObnN4756cG4f3tA5Q+vya7PXxfQ?=
 =?us-ascii?Q?o/u6pJ13ino/4zBR72JGEGfLcnsvH0N8WqwWbDLB7VXoXr4AG+/nkuE3LjxF?=
 =?us-ascii?Q?O43IsCBGR4ItigIX87EtSiLuWIEFUt3vdsKKyB9LYYMjvjEUkmhx9UDjY9DT?=
 =?us-ascii?Q?0Y/EPlRpCK8O7EPHaJhiHjJURWSXCmwNDuzD/got1yIqdAJglwe41KxkU6Ey?=
 =?us-ascii?Q?4v5KdVjnCj2wJ0Ztsf7USjkIJ8HpatmjX6rhIus9axg+Dyvk9unZegEA9xYK?=
 =?us-ascii?Q?dBOSeOF9+5kg/WAGc2zJ2ikxOH9YcRJV7KSU8+0EKMcaLNL+Pl7j4Dl7Uy9T?=
 =?us-ascii?Q?01epgltVJCRtGaDaiHP5QuvfefFkFGrNTQrao1iPQ1abtLhUZbvg/L2mDAzi?=
 =?us-ascii?Q?M/PgCPCH2PhCZ0XIZ+z71TkI5NyOYMpsJdyLoEP48zbp17IUaF2wDUgpiRBc?=
 =?us-ascii?Q?FRroYV8Qe7vWxni2XZqGqxbbXOQdNF4+rgs3mgqnpbSvsBMYstRitulhNW8h?=
 =?us-ascii?Q?+zlU1qhbZb7Oc/QDo8rLb5atiDIVkG350p7POYM7kPnW3LfhPT0HHUQHUVVA?=
 =?us-ascii?Q?O9e3h2Ykw4rMoI5kIRn1F05i6YHJXssEkkCyca6JFEbTrUF0Wek9Mh+b7N5T?=
 =?us-ascii?Q?fO9XfDv+Ok06V98vZKhlcOsT+8NDCFCRyRAjyg19TR88PEYiQ2MvoTw6LE3P?=
 =?us-ascii?Q?373iK79l+0kWHneH3gelfEfuICiFP2K6RuJI2t9FKEaiHTiwshaKgMpTUDDh?=
 =?us-ascii?Q?sfFVwuf/7t6+tnKVQQ1Jdc728R/5qiNtxGl0/3U043+reNbYamxdPbZd+Vsz?=
 =?us-ascii?Q?pQ/BkPALhxoThhzGJkKHSCCTEqlBzc6CYcCP7fxfeRIqW4VZl5xQK+vi6kWV?=
 =?us-ascii?Q?VmKGbYGhzcl2kyCDh1cMSorCGxuaklxEjLAtP0u8JN1tdRExTWgDWPKJfgQF?=
 =?us-ascii?Q?fkgR1J+0a06UDbh4Bs+vlLOujjivkZFYNi7dmGl9t3cJYOQ4Xq/2FsJdE/or?=
 =?us-ascii?Q?8HcvSa5p94stVs+Vkk2Q1DbV4xiUpduJbZlPCoo8assEK0HdH2wlUlHTJ8hU?=
 =?us-ascii?Q?C0lgTHUBpoh04XdYZI73Ta8iOwWKWJXImlUFZ6p7puuazDxOG2aAzfCZFj7w?=
 =?us-ascii?Q?64BrhQJqc3XuRT+ISeErEGXRJOBOAlPoJkLJd9ETbMn+vXXbUJM1D7ABDWAM?=
 =?us-ascii?Q?/t1WG4txGUNKrsy2CVKl1HHtZqdUQqfJhJ1aEnr73cvWhgLGDZx6XMLD2NfL?=
 =?us-ascii?Q?op/Cb7rMeKW8S/5z7sLK0cyhz3waZblLa6hMS6q9?=
X-MS-Exchange-CrossTenant-Network-Message-Id: 1c341f85-7c94-48cf-3002-08dc6825d79f
X-MS-Exchange-CrossTenant-AuthSource: CH3PR11MB8660.namprd11.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 29 Apr 2024 08:24:52.3281
 (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: Vg7SFE13iXj30A5gRtr6mUu3jAgADFfMDm2r2ZhKsafh9C2HeizjQ2v1WmV9LFgvZej8McTtVRk1BRvWspfvXQ==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA1PR11MB8575
X-OriginatorOrg: intel.com

On Thu, Feb 08, 2024 at 01:26:31AM +0800, Xin Li wrote:
>Add kvm_is_fred_enabled() to get if FRED is enabled on a vCPU.
>
>Signed-off-by: Xin Li <xin3.li@intel.com>
>Tested-by: Shan Kang <shan.kang@intel.com>
>---
>
>Change since v1:
>* Explain why it is ok to only check CR4.FRED (Chao Gao).
>---
> arch/x86/kvm/kvm_cache_regs.h | 17 +++++++++++++++++
> 1 file changed, 17 insertions(+)
>
>diff --git a/arch/x86/kvm/kvm_cache_regs.h b/arch/x86/kvm/kvm_cache_regs.h
>index 75eae9c4998a..1d431c703fdf 100644
>--- a/arch/x86/kvm/kvm_cache_regs.h
>+++ b/arch/x86/kvm/kvm_cache_regs.h
>@@ -187,6 +187,23 @@ static __always_inline bool kvm_is_cr4_bit_set(struct kvm_vcpu *vcpu,
> 	return !!kvm_read_cr4_bits(vcpu, cr4_bit);
> }
> 
>+/*
>+ * 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.
Specifically,

1) ..
2) ..