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Message-ID: <20200416155655.GA7155@lakrids.cambridge.arm.com>
Date: Thu, 16 Apr 2020 16:56:56 +0100
From: Mark Rutland <mark.rutland@....com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Arnd Bergmann <arnd@...db.de>,
linux-arm-msm <linux-arm-msm@...r.kernel.org>,
Andy Gross <agross@...nel.org>, Wei Xu <xuwei5@...ilicon.com>,
Bjorn Andersson <bjorn.andersson@...aro.org>,
Masahiro Yamada <yamada.masahiro@...ionext.com>,
Robert Richter <rrichter@...vell.com>,
Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/2] [RFC] arm64: Add dependencies to vendor-specific
errata
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@....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.
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