Re: [PATCH] arm64: Add the arm64.nolse_atomics command line option

From: Marc Zyngier <maz@kernel.org>
To: "Aiqun(Maria) Yu" <quic_aiquny@quicinc.com>
Cc: <will@kernel.org>, <corbet@lwn.net>, <catalin.marinas@arm.com>,
	<quic_pkondeti@quicinc.com>, <quic_kaushalk@quicinc.com>,
	<quic_satyap@quicinc.com>, <quic_shashim@quicinc.com>,
	<quic_songxue@quicinc.com>, <linux-doc@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>,
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] arm64: Add the arm64.nolse_atomics command line option
Date: Tue, 11 Jul 2023 07:57:23 +0100	[thread overview]
Message-ID: <86lefnvsto.wl-maz@kernel.org> (raw)
In-Reply-To: <6e07ad52-2629-346e-6217-ec07777ebc5b@quicinc.com>

On Tue, 11 Jul 2023 04:30:44 +0100,
"Aiqun(Maria) Yu" <quic_aiquny@quicinc.com> wrote:
> 
> On 7/10/2023 5:31 PM, Marc Zyngier wrote:
> > On Mon, 10 Jul 2023 09:19:54 +0100,
> > "Aiqun(Maria) Yu" <quic_aiquny@quicinc.com> wrote:
> >> 
> >> On 7/10/2023 3:27 PM, Marc Zyngier wrote:
> >>> On Mon, 10 Jul 2023 06:59:55 +0100,
> >>> Maria Yu <quic_aiquny@quicinc.com> wrote:
> >>>> 
> >>>> In order to be able to disable lse_atomic even if cpu
> >>>> support it, most likely because of memory controller
> >>>> cannot deal with the lse atomic instructions, use a
> >>>> new idreg override to deal with it.
> >>> 
> >>> In general, the idreg overrides are *not* there to paper over HW bugs.
> >>> They are there to force the kernel to use or disable a feature for
> >>> performance reason or to guide the *enabling* of a feature, but not
> >>> because the HW is broken.
> >>> 
> >>> The broken status of a HW platform must also be documented so that we
> >>> know what to expect when we look at, for example, a bad case of memory
> >>> corruption (something I'd expect to see on a system that only
> >>> partially implements atomic memory operations).
> >>> 
> >> 
> >> good idea. A noc error would be happened if the lse atomic instruction
> >> happened during a memory controller doesn't support lse atomic
> >> instructions.
> >> I can put the information in next patchset comment message. Pls feel
> >> free to let know if there is other place to have this kind of
> >> information with.
> > 
> > For a start, Documentation/arch/arm64/silicon-errata.rst should
> > contain an entry for the actual erratum, and a description of the
> > symptoms of the issue (you're mentioning a "noc error": how is that
> > reported to the CPU?).
> 
> This is not a cpu's errata as my understanding. It is the DDR
> subsystem which don't have the LSE atomic feature supported.

CPU or not doesn't matter. We also track system errata.

> > 
> > The workaround should also be detected at runtime -- we cannot rely on
> > the user to provide a command-line argument to disable an essential
> > feature that anyone has taken for granted for most of a decade...
> 
> We are also seeking help from DDR Subsystem POC to see whether it is
> possible to detect the LSE atomic feature support or not at runtime.

Keying it off a DT compatible (or something similar) would work.

> In my opinion, LSE atomic is a system level feature instead of a cpu
> only feature. So currently solution we is that even if cpu support lse
> atomic, but it still needed to be disabled if the cpu working with a
> lse atomic not support by current system's DDR subsystem.

In the absence of a detection mechanism for anything past the CPU,
this is a moot point. At this stage, this is a bit like saying
"writing to memory is a system thing, not only a CPU feature".

And this also breaks KVM if these CPUs don't have FWB, as a guest can
always map a piece of memory as non-cacheable, and trigger the issue
you describe in your reply to Will, even if you hide the atomics on
the host.

	M.

-- 
Without deviation from the norm, progress is not possible.

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