[RFC PATCH 4/8] x86/fpu/xstate: Introduce kernel dynamic xfeature set

[Yang Weijiang]

Define a new kernel xfeature set including the features can be dynamically
enabled, i.e., the relevant feature is enabled on demand. The xfeature set
is currently used by KVM to configure __guest__ fpstate, i.e., calculating
the xfeature and fpstate storage size etc. The xfeature set is initialized
once and used whenever it's referenced to avoid repeat calculation.

Currently it's used when 1) guest fpstate __state_size is calculated while
guest permits are configured 2) guest vCPU is created and its fpstate is
initialized.

Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
---
 arch/x86/kernel/fpu/xstate.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index c3ed86732d33..eaec05bc1b3c 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -84,6 +84,8 @@ static unsigned int xstate_sizes[XFEATURE_MAX] __ro_after_init =
 	{ [ 0 ... XFEATURE_MAX - 1] = -1};
 static unsigned int xstate_flags[XFEATURE_MAX] __ro_after_init;
 
+u64 fpu_kernel_dynamic_xfeatures __ro_after_init;
+
 #define XSTATE_FLAG_SUPERVISOR	BIT(0)
 #define XSTATE_FLAG_ALIGNED64	BIT(1)
 
@@ -740,6 +742,23 @@ static void __init fpu__init_disable_system_xstate(unsigned int legacy_size)
 	fpstate_reset(&current->thread.fpu);
 }
 
+static unsigned short xsave_kernel_dynamic_xfeatures[] = {
+	[XFEATURE_CET_KERNEL]	= X86_FEATURE_SHSTK,
+};
+
+static void __init init_kernel_dynamic_xfeatures(void)
+{
+	unsigned short cid;
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(xsave_kernel_dynamic_xfeatures); i++) {
+		cid = xsave_kernel_dynamic_xfeatures[i];
+
+		if (cid && boot_cpu_has(cid))
+			fpu_kernel_dynamic_xfeatures |= BIT_ULL(i);
+	}
+}
+
 /*
  * Enable and initialize the xsave feature.
  * Called once per system bootup.
@@ -809,6 +828,8 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 	if (boot_cpu_has(X86_FEATURE_SHSTK) || boot_cpu_has(X86_FEATURE_IBT))
 		fpu_kernel_cfg.max_features |= BIT_ULL(XFEATURE_CET_USER);
 
+	init_kernel_dynamic_xfeatures();
+
 	if (!cpu_feature_enabled(X86_FEATURE_XFD))
 		fpu_kernel_cfg.max_features &= ~XFEATURE_MASK_USER_DYNAMIC;
 
-- 
2.27.0