Re: [PATCH v4 2/4] arm64: Work around Falkor erratum 1003

From: Will Deacon <will.deacon@arm.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: "Mark Rutland" <mark.rutland@arm.com>,
	"Mark Langsdorf" <mlangsdo@redhat.com>,
	"Christoffer Dall" <christoffer.dall@linaro.org>,
	kvm@vger.kernel.org, "Radim Krčmář" <rkrcmar@redhat.com>,
	"Marc Zyngier" <marc.zyngier@arm.com>,
	"Jon Masters" <jcm@redhat.com>,
	timur@codeaurora.org, "Jonathan Corbet" <corbet@lwn.net>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	kvmarm@lists.cs.columbia.edu,
	"Christopher Covington" <cov@codeaurora.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Neil Leeder" <nleeder@codeaurora.org>,
	shankerd@codeaurora.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 2/4] arm64: Work around Falkor erratum 1003
Date: Wed, 1 Feb 2017 17:59:48 +0000	[thread overview]
Message-ID: <20170201175947.GL8177@arm.com> (raw)
In-Reply-To: <20170201174934.GF26507@e104818-lin.cambridge.arm.com>

On Wed, Feb 01, 2017 at 05:49:34PM +0000, Catalin Marinas wrote:
> On Wed, Feb 01, 2017 at 05:41:05PM +0000, Will Deacon wrote:
> > On Wed, Feb 01, 2017 at 05:36:09PM +0000, Catalin Marinas wrote:
> > > On Wed, Feb 01, 2017 at 04:33:58PM +0000, Will Deacon wrote:
> > > > On Wed, Feb 01, 2017 at 11:29:22AM -0500, Christopher Covington wrote:
> > > > > On 01/31/2017 12:56 PM, Marc Zyngier wrote:
> > > > > > Given that all ARMv8 CPUs can support SW_PAN, it is more likely to be
> > > > > > enabled than the ARMv8.1 PAN. I'd vote for supporting the workaround in
> > > > > > that case too, and hope that people do enable the HW version.
> > > > > 
> > > > > Okay, I'll do my best to add support for the SW PAN case. I rebased and
> > > > > submitted v6 of the E1009 patch [1] so that it no longer depends on this
> > > > > patch landing first, if you all are inclined to pick it up while work on
> > > > > this E1003 patch continues.
> > > > 
> > > > The alternative is not enabling SW_PAN (at runtime) if this errata is
> > > > present, along with a warning stating that hardware-PAN should be
> > > > enabled in kconfig instead. Not sure what distributions will make of that
> > > > though.
> > > 
> > > The problem with this patch is that when ARM64_SW_TTBR0_PAN is enabled
> > > and in the absence of hardware PAN (or ARM64_PAN disabled),
> > > cpu_do_switch_mm is no longer called for user process switching, so the
> > > workaround is pretty much useless.
> > 
> > Oh, I see what you mean now.
> > 
> > > I'm ok with adding the Kconfig dependency below to
> > > QCOM_FALKOR_ERRATUM_1003:
> > > 
> > > 	depends on !ARM64_SW_TTBR0_PAN || ARM64_PAN
> > > 
> > > together with a run-time warning if ARM64_SW_TTBR0_PAN is being used.
> > 
> > That makes it look like hardware-PAN is the cause of the erratum.
> 
> With the right Kconfig comment we could make this clearer.

It's not just a comment though, the kconfig option for the workaround
will disappear from menuconfig as long as the dependencies aren't met.
The dependency is really that SW_PAN depends on !ERRATUM_1003, but that
doesn't work for the distributions.

> > Maybe
> > just select ARM64_PAN if the erratum workaround is selected, then
> > runtime warning if we find that the h/w doesn't have PAN but does have
> > the erratum (which should never fire)?
> 
> You still need this workaround even if you don't want any PAN (both sw
> and hw PAN disabled). I wouldn't want to select ARM64_PAN since it's not
> a dependency. It's more like if you do need a PAN, make sure you only
> use the hw one.

True, in the case that all PAN options are disabled we still want this
to work. How about:

  select ARM64_PAN if ARM64_SW_TTBR0_PAN

?

In fact, what's the reason for supporting SW_PAN and ARM64_PAN as a
config combination? Why not just have "PAN" that enables them both and
uses the hardware feature if it's there?

Will