linux-kernel - Re: [PATCH v1 06/24] kvm: arm64: Support per_cpu

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Date:   Wed, 11 Nov 2020 12:32:22 +0000
From:   David Brazdil <dbrazdil@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Dennis Zhou <dennis@...nel.org>,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Quentin Perret <qperret@...gle.com>,
        Andrew Scull <ascull@...gle.com>,
        Andrew Walbran <qwandor@...gle.com>, kernel-team@...roid.com
Subject: Re: [PATCH v1 06/24] kvm: arm64: Support per_cpu_ptr in nVHE hyp code

> > +
> > +	cpu_base_array = kern_hyp_va(&kvm_arm_hyp_percpu_base[0]);
> 
> There is no guarantee that this will not generate a PC relative
> addressing, resulting in kern_hyp_va() being applied twice.
> 
> Consider using hyp_symbol_addr() instead, which always does the right
> by forcing a PC relative addressing and not subsequently mangling
> the address.
> 
> > +	this_cpu_base = kern_hyp_va(cpu_base_array[cpu]);
> > +	return this_cpu_base - (unsigned long)&__per_cpu_start;
> 
> And this is the opposite case: if the compiler generates an absolute
> address, you're toast. Yes, this is just as unlikely, but hey...
> Same remedy should apply.

Good point, and I'll probably keep forgetting about this in the future. Now
that all .hyp.text is only executed under hyp page tables, should we start
thinking about fixing up the relocations?