powerpc/vdso: Implement __arch_get_vdso_rng_data()

VDSO time functions do not call any other function, so they don't
need to save/restore LR. However, retrieving the address of VDSO data
page requires using LR hence saving then restoring it, which can be
heavy on some CPUs. On the other hand, VDSO functions on powerpc are
not standard functions and require a wrapper function to call C VDSO
functions. And that wrapper has to save and restore LR in order to
call the C VDSO function, so retrieving VDSO data page address in that
wrapper doesn't require additional save/restore of LR.

For random VDSO functions it is a bit different. Because the function
calls __arch_chacha20_blocks_nostack(), it saves and restores LR.
Retrieving VDSO data page address can then be done there without
additional save/restore of LR.

So lets implement __arch_get_vdso_rng_data() and simplify the wrapper.

It starts paving the way for the day powerpc will implement a more
standard ABI for VDSO functions.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/a1a9bd0df508f1b5c04684b7366940577dfc6262.1727858295.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/vdso/getrandom.h

@@ -7,6 +7,8 @@
 
 #ifndef __ASSEMBLY__
 
+#include <asm/vdso_datapage.h>
+
 static __always_inline int do_syscall_3(const unsigned long _r0, const unsigned long _r3,
 					const unsigned long _r4, const unsigned long _r5)
 {
@@ -43,11 +45,21 @@ static __always_inline ssize_t getrandom_syscall(void *buffer, size_t len, unsig
 
 static __always_inline struct vdso_rng_data *__arch_get_vdso_rng_data(void)
 {
-	return NULL;
+	struct vdso_arch_data *data;
+
+	asm (
+		"	bcl	20, 31, .+4 ;"
+		"0:	mflr	%0 ;"
+		"	addis	%0, %0, (_vdso_datapage - 0b)@ha ;"
+		"	addi	%0, %0, (_vdso_datapage - 0b)@l  ;"
+		: "=r" (data) : : "lr"
+	);
+
+	return &data->rng_data;
 }
 
 ssize_t __c_kernel_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state,
-			     size_t opaque_len, const struct vdso_rng_data *vd);
+			     size_t opaque_len);
 
 #endif /* !__ASSEMBLY__ */
 

arch/powerpc/kernel/asm-offsets.c

@@ -335,7 +335,6 @@ int main(void)
 
 	/* datapage offsets for use by vdso */
 	OFFSET(VDSO_DATA_OFFSET, vdso_arch_data, data);
-	OFFSET(VDSO_RNG_DATA_OFFSET, vdso_arch_data, rng_data);
 	OFFSET(CFG_TB_TICKS_PER_SEC, vdso_arch_data, tb_ticks_per_sec);
 #ifdef CONFIG_PPC64
 	OFFSET(CFG_ICACHE_BLOCKSZ, vdso_arch_data, icache_block_size);

arch/powerpc/kernel/vdso/getrandom.S

@@ -31,7 +31,6 @@
 	PPC_STL		r2, PPC_MIN_STKFRM + STK_GOT(r1)
   .cfi_rel_offset r2, PPC_MIN_STKFRM + STK_GOT
 #endif
-	get_datapage	r8 VDSO_RNG_DATA_OFFSET
 	bl		CFUNC(DOTSYM(\funct))
 	PPC_LL		r0, PPC_MIN_STKFRM + PPC_LR_STKOFF(r1)
 #ifdef __powerpc64__

arch/powerpc/kernel/vdso/vgetrandom.c

@@ -8,7 +8,7 @@
 #include <linux/types.h>
 
 ssize_t __c_kernel_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state,
-			     size_t opaque_len, const struct vdso_rng_data *vd)
+			     size_t opaque_len)
 {
-	return __cvdso_getrandom_data(vd, buffer, len, flags, opaque_state, opaque_len);
+	return __cvdso_getrandom(buffer, len, flags, opaque_state, opaque_len);
 }