x86/fpu: Initialize guest FPU permissions from guest defaults

Currently, fpu->guest_perm is copied from fpu->perm, which is derived from
fpu_kernel_cfg.default_features.

Guest defaults were introduced to differentiate the features and sizes of
host and guest FPUs. Copying guest FPU permissions from the host will lead
to inconsistencies between the guest default features and permissions.

Initialize guest FPU permissions from guest defaults instead of host
defaults. This ensures that any changes to guest default features are
automatically reflected in guest permissions, which in turn guarantees
that fpstate_realloc() allocates a correctly sized XSAVE buffer for guest
FPUs.

Suggested-by: Chang S. Bae <[email protected]>
Signed-off-by: Chao Gao <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Rick Edgecombe <[email protected]>
Reviewed-by: John Allen <[email protected]>
Link: https://lore.kernel.org/all/20250522151031.426788-3-chao.gao%40intel.com

Author: gaochaointel

arch/x86/kernel/fpu/core.c

@@ -562,8 +562,14 @@ void fpstate_reset(struct fpu *fpu)
 	fpu->perm.__state_perm		= fpu_kernel_cfg.default_features;
 	fpu->perm.__state_size		= fpu_kernel_cfg.default_size;
 	fpu->perm.__user_state_size	= fpu_user_cfg.default_size;
-	/* Same defaults for guests */
-	fpu->guest_perm = fpu->perm;
+
+	fpu->guest_perm.__state_perm	= guest_default_cfg.features;
+	fpu->guest_perm.__state_size	= guest_default_cfg.size;
+	/*
+	 * User features and sizes are always identical between host and
+	 * guest FPUs, which allows for common guest and userspace ABI.
+	 */
+	fpu->guest_perm.__user_state_size = fpu_user_cfg.default_size;
 }
 
 static inline void fpu_inherit_perms(struct fpu *dst_fpu)