linux-kernel - Re: [PATCH v3 05/15] arm64: kvm: Build hyp-entry.S separately for VHE/nVHE

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Date:   Mon, 22 Jun 2020 11:20:41 +0100
From:   David Brazdil <dbrazdil@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, android-kvm@...gle.com,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 05/15] arm64: kvm: Build hyp-entry.S separately for
 VHE/nVHE

Hi Marc,

> > -	void *dst = lm_alias(__bp_harden_hyp_vecs + slot * SZ_2K);
> > +	char *vec = has_vhe() ? __bp_harden_hyp_vecs
> > +			      : kvm_nvhe_sym(__bp_harden_hyp_vecs);
> 
> If we get this construct often, then something that abstracts
> the uggliness of the symbol duality would be nice...

Agreed, I do hope that this will end up being limited to finding the address of
the hyp-init vector once EL2 becomes self-contained. Even this vector selection
can be done in EL2 where the symbol duality does not exist.
If we were to hide it, there is a trade off between code "elegance" and clarity
of what's happening under the hood. I was thinking we could extract this
`has_vhe() ? foo : __kvm_nvhe_foo` as a `#define foo` but I do worry that
anybody debugging this code would be cursing my name. It would also not work
with other macros that take symbol names, notably kvm_ksym_ref. But that can be
rewritten to accept a pointer instead. The more verbose but less magic approach
is to have a bunch of different helpers for various situations, eg.
__pa_symbol_nvhe. What would be your preference?

-David