Le 07/07/2020 à 13:25, Mathieu Desnoyers a écrit :

----- On Jul 7, 2020, at 1:50 AM, Nicholas Piggin npiggin@gmail.com wrote:

quoted

Excerpts from Christophe Leroy's message of July 6, 2020 7:53 pm:

quoted

Le 06/07/2020 à 04:18, Nicholas Piggin a écrit :

quoted

diff --git a/arch/powerpc/include/asm/exception-64s.h

b/arch/powerpc/include/asm/exception-64s.h
index 47bd4ea0837d..b88cb3a989b6 100644

--- a/arch/powerpc/include/asm/exception-64s.h
+++ b/arch/powerpc/include/asm/exception-64s.h

@@ -68,6 +68,10 @@
    *
    * The nop instructions allow us to insert one or more instructions to flush the
    * L1-D cache when returning to userspace or a guest.
+ *
+ * powerpc relies on return from interrupt/syscall being context synchronising
+ * (which hrfid, rfid, and rfscv are) to support ARCH_HAS_MEMBARRIER_SYNC_CORE
+ * without additional additional synchronisation instructions.

This file is dedicated to BOOK3S/64. What about other ones ?

On 32 bits, this is also valid as 'rfi' is also context synchronising,
but then why just add some comment in exception-64s.h and only there ?

Yeah you're right, I basically wanted to keep a note there just in case,
because it's possible we would get a less synchronising return (maybe
unlikely with meltdown) or even return from a kernel interrupt using a
something faster (e.g., bctar if we don't use tar register in the kernel
anywhere).

So I wonder where to add the note, entry_32.S and 64e.h as well?

For 64-bit powerpc, I would be tempted to either place the comment in the header
implementing the RFI_TO_USER and RFI_TO_USER_OR_KERNEL macros or the .S files
using them, e.g. either:

arch/powerpc/include/asm/exception-64e.h
arch/powerpc/include/asm/exception-64s.h

or

arch/powerpc/kernel/exceptions-64s.S
arch/powerpc/kernel/entry_64.S

And for 32-bit powerpc, AFAIU

arch/powerpc/kernel/entry_32.S

uses SYNC + RFI to return to user-space. RFI is defined in

arch/powerpc/include/asm/ppc_asm.h

So a comment either near the RFI define and its uses should work.

For 32-bit, RFI is likely to go away the day 40x goes away, so I 
wouldn't put it there.
Places like head_8xx.S use rfi not RFI.

And the SYNC is about to go when we decide to retire 601 SYNC FIX.

So it would be probably better to put it somewhere in entry_32.S

Christophe