Re: [PATCH 2/2] [RFC] arm64: Add dependencies to vendor-specific errata

From: Mark Rutland <mark.rutland@arm.com>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	Andy Gross <agross@kernel.org>, Wei Xu <xuwei5@hisilicon.com>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Robert Richter <rrichter@marvell.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 2/2] [RFC] arm64: Add dependencies to vendor-specific errata
Date: Thu, 16 Apr 2020 16:56:56 +0100	[thread overview]
Message-ID: <20200416155655.GA7155@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <CAMuHMdWRW4+YLR8fz0hUTAPupRkM4Y5c82XHuOWSvNYOh-BZ0A@mail.gmail.com>

On Thu, Apr 16, 2020 at 05:38:07PM +0200, Geert Uytterhoeven wrote:
> On Thu, Apr 16, 2020 at 2:56 PM Mark Rutland <mark.rutland@arm.com> wrote:
> > On Thu, Apr 16, 2020 at 01:56:58PM +0200, Geert Uytterhoeven wrote:
> > > Currently the user is asked about enabling support for each and every
> > > vendor-specific erratum, even when support for the specific platform is
> > > not enabled.
> > >
> > > Fix this by adding platform dependencies to the config options
> > > controlling support for vendor-specific errata.

> > I'm not su1re that it makes sense to do this in general, becaose the
> > ARCH_* platform symbols are about plactform/SoC support (e.g. pinctrl
> > drivers), and these are (mostly) CPU-local and/or VM-visible.
> >
> > I think that it makes sense for those to be independent because:

> > * It prevents building a minimal VM image with all (non-virtualized)
> >   platform support disabled, but all possible (VM-visible) errata
> >   options enabled. I do that occassionally for testing/analysis, and I
> >   can imagine this is useful for those building images that are only
> >   intended to be used in VMs.
> 
> Oh, you also want to build a "generic" guest kernel, with all ARCH_*
> symbols disabled. 

Yup! As above I do this today for building test kernels I run on a
number of different hosts, and I'm aware of other use-cases (e.g. WSL2
or docker for mac) where you may want to do this to minimize the core
kernel either for size or security reasons.

> Let's hope a maleficent user cannot disable errata mitigations in the
> guest kernel and break the host ;-)

Indeed ;)

For cases where a malicious guest could cause harm we've added
workarounds in KVM, so unless we've missed something that shouldn't be
the case.

Otherwise, a guest missing these is just shooting itself in the foot.

> And perhaps you do want to enable some platform-specific drivers for
> VFIO pass-through?  Hence having ARCH_* dependencies on those drivers
> means they cannot be enabled :-( Hmm...

IIRC platform device passthrough requires an corresponding VFIO platform
driver in the host to handle reset and so on, but it does seem a shame
to not allow the user to select a driver if they really want it.

I guess there might be platform-specific PCIe drivers too, which might
work with VFIO regardless.

Thanks,
Mark.