linux-kernel - [PATCH v4 7/8] x86/fpu/xstate: Introduce "guest-only" supervisor xfeature set

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Message-ID: <20250318153316.1970147-8-chao.gao@intel.com>
Date: Tue, 18 Mar 2025 23:31:57 +0800
From: Chao Gao <chao.gao@...el.com>
To: x86@...nel.org,
	linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org,
	tglx@...utronix.de,
	dave.hansen@...el.com,
	seanjc@...gle.com,
	pbonzini@...hat.com
Cc: peterz@...radead.org,
	rick.p.edgecombe@...el.com,
	weijiang.yang@...el.com,
	john.allen@....com,
	bp@...en8.de,
	chang.seok.bae@...el.com,
	xin3.li@...el.com,
	Chao Gao <chao.gao@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Maxim Levitsky <mlevitsk@...hat.com>,
	Samuel Holland <samuel.holland@...ive.com>,
	Mitchell Levy <levymitchell0@...il.com>,
	Li RongQing <lirongqing@...du.com>,
	Vignesh Balasubramanian <vigbalas@....com>,
	Aruna Ramakrishna <aruna.ramakrishna@...cle.com>
Subject: [PATCH v4 7/8] x86/fpu/xstate: Introduce "guest-only" supervisor xfeature set

From: Yang Weijiang <weijiang.yang@...el.com>

Define a new XFEATURE_MASK_SUPERVISOR_GUEST mask to specify the features
that are enabled by default in guest FPUs but not in host FPUs.

Add CET_KERNEL as the first guest-only feature to save host FPUs from
allocating XSAVE buffer space for all threads on CET-capable parts.

Co-developed-by: Chao Gao <chao.gao@...el.com>
Signed-off-by: Chao Gao <chao.gao@...el.com>
Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
---
Dropped Dave's Suggested-by as the patch has been changed significantly
---
 arch/x86/include/asm/fpu/types.h  | 9 +++++----
 arch/x86/include/asm/fpu/xstate.h | 3 +++
 arch/x86/kernel/fpu/xstate.c      | 5 ++++-
 3 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index 80647c060b32..079f3241e25b 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -568,8 +568,9 @@ struct fpu_state_config {
 	 * @default_size:
 	 *
 	 * The default size of the register state buffer. Includes all
-	 * supported features except independent managed features and
-	 * features which have to be requested by user space before usage.
+	 * supported features except independent managed features,
+	 * guest-only features and features which have to be requested by
+	 * user space before usage.
 	 */
 	unsigned int		default_size;
 
@@ -595,8 +596,8 @@ struct fpu_state_config {
 	 * @default_features:
 	 *
 	 * The default supported features bitmap. Does not include
-	 * independent managed features and features which have to
-	 * be requested by user space before usage.
+	 * independent managed features, guest-only features and features
+	 * which have to be requested by user space before usage.
 	 */
 	u64 default_features;
 
diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index 8990cf381bef..69db17476061 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -45,6 +45,9 @@
 /* Features which are dynamically enabled for a process on request */
 #define XFEATURE_MASK_USER_DYNAMIC	XFEATURE_MASK_XTILE_DATA
 
+/* Supervisor features which are enabled only in guest FPUs */
+#define XFEATURE_MASK_SUPERVISOR_GUEST	XFEATURE_MASK_CET_KERNEL
+
 /* All currently supported supervisor features */
 #define XFEATURE_MASK_SUPERVISOR_SUPPORTED (XFEATURE_MASK_PASID | \
 					    XFEATURE_MASK_CET_USER | \
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 1dd6ddba8723..b19960215074 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -55,7 +55,7 @@ static const char *xfeature_names[] =
 	"Protection Keys User registers",
 	"PASID state",
 	"Control-flow User registers",
-	"Control-flow Kernel registers",
+	"Control-flow Kernel registers (KVM only)",
 	"unknown xstate feature",
 	"unknown xstate feature",
 	"unknown xstate feature",
@@ -813,6 +813,9 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 	fpu_kernel_cfg.default_features &= ~XFEATURE_MASK_USER_DYNAMIC;
 	fpu_kernel_cfg.guest_default_features = fpu_kernel_cfg.default_features;
 
+	/* Clean out guest-only features from default */
+	fpu_kernel_cfg.default_features &= ~XFEATURE_MASK_SUPERVISOR_GUEST;
+
 	fpu_user_cfg.default_features = fpu_user_cfg.max_features;
 	fpu_user_cfg.default_features &= ~XFEATURE_MASK_USER_DYNAMIC;
 	fpu_user_cfg.guest_default_features = fpu_user_cfg.default_features;
-- 
2.46.1