Re: [PATCH 3/6] arm64: scs: Use 'scs_sp' register alias for x18

From: Will Deacon <will@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-kernel@vger.kernel.org,
	Sami Tolvanen <samitolvanen@google.com>,
	kernel-team@android.com, Ard Biesheuvel <ardb@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 3/6] arm64: scs: Use 'scs_sp' register alias for x18
Date: Mon, 18 May 2020 14:03:36 +0100	[thread overview]
Message-ID: <20200518130335.GC32394@willie-the-truck> (raw)
In-Reply-To: <20200518115547.GC1957@C02TD0UTHF1T.local>

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 <will@kernel.org>
> 
> 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 <mark.rutland@arm.com>
> 
> 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.

Will

--->8

From 7e86208cd6541c1229bc1fcd206596308d1727f8 Mon Sep 17 00:00:00 2001
From: Will Deacon <will@kernel.org>
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 <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
 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.
  *
  * We save the callsite's context into a pt_regs before invoking any ftrace
  * callbacks. So that we can get a sensible backtrace, we create a stack record
-- 
2.26.2.761.g0e0b3e54be-goog

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