----- On Jul 7, 2020, at 1:50 AM, Nicholas Piggin npiggin@xxxxxxxxx wrote: > Excerpts from Christophe Leroy's message of July 6, 2020 7:53 pm: >> >> >> Le 06/07/2020 à 04:18, Nicholas Piggin a écrit : >>> 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. > I should actually change the comment for 64-bit because soft masked > interrupt replay is an interesting case. I thought it was okay (because > the IPI would cause a hard interrupt which does do the rfi) but that > should at least be written. Yes. > The context synchronisation happens before > the Linux IPI function is called, but for the purpose of membarrier I > think that is okay (the membarrier just needs to have caused a memory > barrier + context synchronistaion by the time it has done). Can you point me to the code implementing this logic ? Thanks, Mathieu > > Thanks, > Nick -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
