powerpc/rtas: PACA can be restored directly from SPRG

On 64-bit, PACA is saved in a SPRG so it does not need to be saved on
stack. We also don't need to mask off the top bits for real mode
addresses because the architecture does this for us.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Laurent Dufour <ldufour@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220308135047.478297-8-npiggin@gmail.com

arch/powerpc/kernel/rtas_entry.S

@@ -62,10 +62,9 @@ _GLOBAL(enter_rtas)
 
 	/* Because RTAS is running in 32b mode, it clobbers the high order half
 	 * of all registers that it saves.  We therefore save those registers
-	 * RTAS might touch to the stack.  (r0, r3-r13 are caller saved)
+	 * RTAS might touch to the stack.  (r0, r3-r12 are caller saved)
 	 */
 	SAVE_GPR(2, r1)			/* Save the TOC */
-	SAVE_GPR(13, r1)		/* Save paca */
 	SAVE_NVGPRS(r1)			/* Save the non-volatiles */
 
 	mfcr	r4
@@ -135,15 +134,14 @@ rtas_return_loc:
 	mtmsrd	r6
 
 	/* relocation is off at this point */
-	GET_PACA(r4)
-	clrldi	r4,r4,2			/* convert to realmode address */
+	GET_PACA(r13)
 
 	bcl	20,31,$+4
 0:	mflr	r3
 	ld	r3,(1f-0b)(r3)		/* get &rtas_restore_regs */
 
-	ld	r1,PACAR1(r4)		/* Restore our SP */
-	ld	r4,PACASAVEDMSR(r4)	/* Restore our MSR */
+	ld	r1,PACAR1(r13)		/* Restore our SP */
+	ld	r4,PACASAVEDMSR(r13)	/* Restore our MSR */
 
 	mtspr	SPRN_SRR0,r3
 	mtspr	SPRN_SRR1,r4
@@ -159,11 +157,8 @@ _ASM_NOKPROBE_SYMBOL(rtas_return_loc)
 rtas_restore_regs:
 	/* relocation is on at this point */
 	REST_GPR(2, r1)			/* Restore the TOC */
-	REST_GPR(13, r1)		/* Restore paca */
 	REST_NVGPRS(r1)			/* Restore the non-volatiles */
 
-	GET_PACA(r13)
-
 	ld	r4,_CCR(r1)
 	mtcr	r4
 	ld	r5,_CTR(r1)