Re: [PATCH] KVM: arm64: Cleanup the __get_fault_info() to take out the code that validates HPFAR

From: Marc Zyngier <maz@kernel.org>
To: Mingwei Zhang <mizhang@google.com>
Cc: Oliver Upton <oliver.upton@linux.dev>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	"moderated list:KERNEL VIRTUAL MACHINE FOR ARM64 (KVM/arm64)"
	<kvmarm@lists.cs.columbia.edu>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] KVM: arm64: Cleanup the __get_fault_info() to take out the code that validates HPFAR
Date: Wed, 28 Sep 2022 06:40:03 -0400	[thread overview]
Message-ID: <86pmff7pfg.wl-maz@kernel.org> (raw)
In-Reply-To: <CAL715W+gJKH+3xgFzUjPs6SAMwZCzkF5NNOTDpa4ov2qZ3r_iA@mail.gmail.com>

Mingwei,

On Tue, 27 Sep 2022 13:48:52 -0400,
Mingwei Zhang <mizhang@google.com> wrote:
> 
> >
> > Honestly, I'd refrain from such changes *unless* they enable something
> > else. The current code is well understood by people hacking on it, and
> > although I don't mind revamping it, it has to be for a good reason.
> >
> > I'd be much more receptive to such a change if it was a prefix to
> > something that actually made a significant change.
> >
> > Thanks,
> >
> >         M.
> >
> Hi Marc,
> 
> Thanks for the feedback.  I am not sure about the style of the KVM ARM
> side. But in general I think mixing the generic code for ARM and
> specific CPU errata handling is misleading. For instance, in this
> case:
> 
> +     if ((esr & ESR_ELx_FSC_TYPE) == FSC_PERM)
> +             return false;
> +
> +     if (cpus_have_final_cap(ARM64_WORKAROUND_834220))
> +             return false;
> 
> As shown it would be much cleaner to separate the two cases as the
> former case is suggested in ARMv8 Spec D13.2.55. The latter case would
> definitely come from a different source.

I think we're talking at cross purposes. I don't object to the change
per se. I simply question its value *in isolation*. One of the many
things that makes the kernel hard to maintain is churn. Refactoring
just for the sake of it *is* churn. In this case, cosmetic churn.

But if you make this is part of something touching this area and
improving things from a functional perspective, then I'll happily
merge it.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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