linux-kernel - Re: [PATCH 5.1 51/55] bpf, arm64: use more scalable stadd over ldxr / stxr loop in xadd

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Date:   Wed, 3 Jul 2019 09:24:16 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        Will Deacon <will.deacon@....com>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH 5.1 51/55] bpf, arm64: use more scalable stadd over ldxr
 / stxr loop in xadd

On Tue, Jul 02, 2019 at 10:02:00PM -0400, Sasha Levin wrote:
> On Tue, Jul 02, 2019 at 10:01:59AM +0200, Greg Kroah-Hartman wrote:
> > From: Daniel Borkmann <daniel@...earbox.net>
> > 
> > commit 34b8ab091f9ef57a2bb3c8c8359a0a03a8abf2f9 upstream.
> > 
> > Since ARMv8.1 supplement introduced LSE atomic instructions back in 2016,
> > lets add support for STADD and use that in favor of LDXR / STXR loop for
> > the XADD mapping if available. STADD is encoded as an alias for LDADD with
> > XZR as the destination register, therefore add LDADD to the instruction
> > encoder along with STADD as special case and use it in the JIT for CPUs
> > that advertise LSE atomics in CPUID register. If immediate offset in the
> > BPF XADD insn is 0, then use dst register directly instead of temporary
> > one.
> > 
> > Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> > Acked-by: Jean-Philippe Brucker <jean-philippe.brucker@....com>
> > Acked-by: Will Deacon <will.deacon@....com>
> > Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> 
> This one has a fix upstream: c5e2edeb01ae9ffbdde95bdcdb6d3614ba1eb195
> ("arm64: insn: Fix ldadd instruction encoding").

Good catch, now queued up, thanks.

greg k-h