On Mon, May 18, 2020 at 02:03:36PM +0100, Will Deacon wrote: > On Mon, May 18, 2020 at 12:55:47PM +0100, Mark Rutland wrote: > > On Fri, May 15, 2020 at 06:27:53PM +0100, Will Deacon wrote: > > > x18 holds the SCS stack pointer value, so introduce a register alias to > > > make this easier to read in assembly code. > > > > > > Signed-off-by: Will Deacon <[email protected]> > > > > I scanned through arm64 for all instances of x18, and it looks like > > you've covered all the relevant uses here. In kvm we save/restore x18 a > > bunch becasue it might be a platform register, but we do that > > unconditionally and without knowledge of what it contains, so I think > > that's fine to leave as-is. Therefore: > > > > Reviewed-by: Mark Rutland <[email protected]> > > > > As an aside, the comment in entry-ftrace.S is now stale where it says > > that x18 is safe to clobber. I can send a patch to clean that up, unless > > you want to do that yourself. > > Thanks, I'll fix that up (see below). Also, apologies for typo'ing your > email address when I sent this out on Friday.
No worries; I'd already forgotten! > > Will > > --->8 > > From 7e86208cd6541c1229bc1fcd206596308d1727f8 Mon Sep 17 00:00:00 2001 > From: Will Deacon <[email protected]> > Date: Mon, 18 May 2020 14:01:01 +0100 > Subject: [PATCH] arm64: entry-ftrace.S: Update comment to indicate that x18 is > live > > The Shadow Call Stack pointer is held in x18, so update the ftrace > entry comment to indicate that it cannot be safely clobbered. > > Reported-by: Mark Rutland <[email protected]> > Signed-off-by: Will Deacon <[email protected]> > --- > arch/arm64/kernel/entry-ftrace.S | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/arch/arm64/kernel/entry-ftrace.S > b/arch/arm64/kernel/entry-ftrace.S > index 833d48c9acb5..a338f40e64d3 100644 > --- a/arch/arm64/kernel/entry-ftrace.S > +++ b/arch/arm64/kernel/entry-ftrace.S > @@ -23,8 +23,9 @@ > * > * ... where <entry> is either ftrace_caller or ftrace_regs_caller. > * > - * Each instrumented function follows the AAPCS, so here x0-x8 and x19-x30 > are > - * live, and x9-x18 are safe to clobber. > + * Each instrumented function follows the AAPCS, so here x0-x8 and x18-x30 > are > + * live (x18 holds the Shadow Call Stack pointer), and x9-x17 are safe to > + * clobber. I'd have called out x18 a bit more specifically: | Each instrumented function follows the AAPCS, so here x0-x8 and x19-x30 are | live, x18 is potentially live with a shadow call stack pointer, and | x9-x17 are safe to clobber. ... but either way this looks good; thanks! Mark.
