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

From: Catalin Marinas <catalin.marinas@arm.com>
To: Will Deacon <will.deacon@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 18:38:12 +0000	[thread overview]
Message-ID: <20170201183812.GI26507@e104818-lin.cambridge.arm.com> (raw)
In-Reply-To: <20170201183400.GA31652@arm.com>

On Wed, Feb 01, 2017 at 06:34:01PM +0000, Will Deacon wrote:
> On Wed, Feb 01, 2017 at 06:22:44PM +0000, Catalin Marinas wrote:
> > On Wed, Feb 01, 2017 at 05:59:48PM +0000, Will Deacon wrote:
> > > 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:
> > > > > 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
> > 
> > As I replied to myself, the above would work for me as well, so let's go
> > for this.
> > 
> > > 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?
> > 
> > Because SW PAN has a non-trivial performance hit. You would enable SW
> > PAN only if you are paranoid about security. HW PAN, OTOH, is very cheap
> > and I wouldn't want to miss enabling it in a single Image supporting
> > ARMv8.0 and ARMv8.1 just because SW PAN is slow on ARMv8.0.
> > 
> > IOW, ARM64_PAN is default y while ARM64_SW_TTBR0_PAN is default n.
> 
> Ok, in that case, then how about another permutation: we make
> ARM64_SW_TTBR0_PAN depend on ARM64_PAN? Then when you select "PAN Support"
> you get a new menu option underneath it for the emulation? I think that
> solves the erratum case and the use-case above.

The problem is that ARM64_PAN is an ARMv8.1 feature and currently
grouped accordingly in Kconfig. ARM64_SW_TTBR0_PAN is complementary (and
even not recommended on ARMv8.1). We can do this if we break the ARMv8.x
feature grouping but just for this erratum, I don't think it's worth.

-- 
Catalin