Michael Ellerman [off-list ref] writes:

quoted

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 9581906b5ee9..65cb14b56f8d 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S

@@ -330,22 +330,18 @@ _GLOBAL(enter_rtas)
 	clrldi	r4,r4,2			/* convert to realmode address */
        	mtlr	r4
 
-	li	r0,0
-	ori	r0,r0,MSR_EE|MSR_SE|MSR_BE|MSR_RI
-	andc	r0,r6,r0
-	
-        li      r9,1
-        rldicr  r9,r9,MSR_SF_LG,(63-MSR_SF_LG)
-	ori	r9,r9,MSR_IR|MSR_DR|MSR_FE0|MSR_FE1|MSR_FP|MSR_RI|MSR_LE
-	andc	r6,r0,r9

 
One advantage of the old method is it can adapt to new MSR bits being
set by the kernel.

For example we used to use RTAS on powernv, and this code didn't need
updating to cater to MSR_HV being set. We will probably never use RTAS
on bare-metal again, so that's OK.

But your change might break secure virtual machines, because it clears
MSR_S whereas the old code didn't. I think SVMs did use RTAS, but I
don't know whether it matters if it's called with MSR_S set or not?

Not sure if anyone will remember, or has a working setup they can test.
Maybe for now we just copy MSR_S from the kernel MSR the way the
current code does.

Would the kernel even be able to change the bit? I think only urfid can
clear MSR_S.