linux-kernel - Re: [RFC PATCH v2 13/26] KVM: arm64: Enable access to sanitized CPU features at EL2

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Message-ID: <X/872aaC/E4MwMa6@google.com>
Date:   Wed, 13 Jan 2021 18:28:41 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, android-kvm@...gle.com,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        Fuad Tabba <tabba@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        David Brazdil <dbrazdil@...gle.com>
Subject: Re: [RFC PATCH v2 13/26] KVM: arm64: Enable access to sanitized CPU
 features at EL2

On Wednesday 13 Jan 2021 at 17:27:49 (+0000), Marc Zyngier wrote:
> On 2021-01-13 14:35, Quentin Perret wrote:
> > On Wednesday 13 Jan 2021 at 14:23:03 (+0000), Quentin Perret wrote:
> > > Good point, that would be nice indeed. Can I use that from outside an
> > > __init function?
> > 
> > Just gave it a go, and the answer to this appears to be yes,
> > surprisingly -- I was expecting a compile-time warning similar to what
> > we get when non-__init code calls into __init, but that doesn't seem to
> > trigger here. Anyways, I'll add the annotation in v3.
> 
> That's surprising. I'd definitely expect something to explode...
> Do you have CONFIG_DEBUG_SECTION_MISMATCH=y?

Yes I do, so, that doesn't seem to be it. Now, the plot thickens: I
_do_ get a warning if I remove the 'const' qualifier. But interestingly,
in both cases hyp_ftr_regs is placed in .init.data:

  $ objdump -t vmlinux | grep hyp_ftr_regs
  ffff8000116c17b0 g     O .init.data     0000000000000030 hyp_ftr_regs

The warning is silenced only if I mark hyp_ftr_regs as const. modpost
bug? I'll double check my findings and follow up in a separate series.

Thanks,
Quentin