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

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Date:   Wed, 1 Feb 2017 18:38:12 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Mark Langsdorf <mlangsdo@...hat.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Jon Masters <jcm@...hat.com>, timur@...eaurora.org,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        Christopher Covington <cov@...eaurora.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Neil Leeder <nleeder@...eaurora.org>, shankerd@...eaurora.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 2/4] arm64: Work around Falkor erratum 1003

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