x86/vdso: Access rng data from kernel without vvar

Remove the usage of the vvar _vdso_rng_data from the kernel-space code,
as the x86 vvar machinery is about to be removed.
The definition of the structure is unnecessary, as the data lives in a
page pre-allocated by the linker anyways.
The vdso user-space access to the rng data will be switched soon.

DEFINE_VVAR_SINGLE() is now unused. It will be removed later togehter
with the rest of vvar.h.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241010-vdso-generic-base-v1-13-b64f0842d512@linutronix.de

Author: t-8ch

arch/x86/entry/vdso/vma.c

@@ -39,7 +39,6 @@ struct vdso_data *arch_get_vdso_data(void *vvar_page)
 #undef EMIT_VVAR
 
 DEFINE_VVAR(struct vdso_data, _vdso_data);
-DEFINE_VVAR_SINGLE(struct vdso_rng_data, _vdso_rng_data);
 
 unsigned int vclocks_used __read_mostly;
 

arch/x86/include/asm/vdso/vsyscall.h

@@ -21,7 +21,7 @@ struct vdso_data *__x86_get_k_vdso_data(void)
 static __always_inline
 struct vdso_rng_data *__x86_get_k_vdso_rng_data(void)
 {
-	return &_vdso_rng_data;
+	return (void *)&__vvar_page + __VDSO_RND_DATA_OFFSET;
 }
 #define __arch_get_k_vdso_rng_data __x86_get_k_vdso_rng_data
 

arch/x86/include/asm/vvar.h

@@ -19,6 +19,8 @@
 #ifndef _ASM_X86_VVAR_H
 #define _ASM_X86_VVAR_H
 
+#define __VDSO_RND_DATA_OFFSET  640
+
 #ifdef EMIT_VVAR
 /*
  * EMIT_VVAR() is used by the kernel linker script to put vvars in the
@@ -62,7 +64,7 @@ DECLARE_VVAR(0, struct vdso_data, _vdso_data)
 
 #if !defined(_SINGLE_DATA)
 #define _SINGLE_DATA
-DECLARE_VVAR_SINGLE(640, struct vdso_rng_data, _vdso_rng_data)
+DECLARE_VVAR_SINGLE(__VDSO_RND_DATA_OFFSET, struct vdso_rng_data, _vdso_rng_data)
 #endif
 
 #undef DECLARE_VVAR