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

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Date:   Wed, 11 Nov 2020 12:46:30 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     David Brazdil <dbrazdil@...gle.com>
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

On 2020-11-11 12:32, David Brazdil wrote:
>> > +
>> > +	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?

Why not, if you can deal with the hypervisor text being mapped at a 
random
location, and make sure that the kernel doesn't process the relocations
for you. This would certainly save us a lot of runtime offsetting (which
I'm adding to in a separate series).

         M.
-- 
Jazz is not dead. It just smells funny...