linux-kernel - Re: [PATCH v1 07/24] kvm: arm64: Create nVHE copy of cpu_logical

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Date:   Wed, 11 Nov 2020 13:29:29 +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 07/24] kvm: arm64: Create nVHE copy of cpu_logical_map

On 2020-11-11 13:03, David Brazdil wrote:
>> > +/*
>> > + * nVHE copy of data structures tracking available CPU cores.
>> > + * Only entries for CPUs that were online at KVM init are populated.
>> > + * Other CPUs should not be allowed to boot because their features were
>> > + * not checked against the finalized system capabilities.
>> > + */
>> > +u64 __ro_after_init __cpu_logical_map[NR_CPUS] = { [0 ... NR_CPUS-1]
>> > = INVALID_HWID };
>> 
>> I'm not sure what __ro_after_init means once we get S2 isolation.
> 
> It is stretching the definition of 'init' a bit, I know, but I don't 
> see what
> your worry is about S2? The intention is to mark this read-only for 
> .hyp.text
> at runtime. With S2, the host won't be able to write to it after KVM 
> init.
> Obviously that's currently not the case.

More importantly, EL2 can write to it at any time, which is the bit I'm 
worried
about, as it makes the annotation misleading.

> One thing we might change in the future is marking it RW for some 
> initial
> setup in a HVC handler, then marking it RO for the rest of uptime.

That'd be a desirable outcome, and it would be consistent with the rest
of the kernel.

> 
>> 
>> > +
>> > +u64 cpu_logical_map(int cpu)
>> 
>> nit: is there any reason why "cpu" cannot be unsigned? The thought
>> of a negative CPU number makes me shiver...
> 
> Same here. That's how it's defined in kernel proper, so I went with 
> that.

I'm happy to deviate from the kernel (give the function a different name
if this clashes with existing include files).

We can also fix the rest of the kernel (I've just written the trivial 
patch).

>> 
>> > +{
>> > +	if (cpu < 0 || cpu >= ARRAY_SIZE(__cpu_logical_map))
>> > +		hyp_panic();
>> > +
>> > +	return __cpu_logical_map[cpu];
>> > +}
>> > +
>> >  unsigned long __hyp_per_cpu_offset(unsigned int cpu)
>> >  {
>> >  	unsigned long *cpu_base_array;
>> 
>> Overall, this patch would make more sense closer it its use case
>> (in patch 19). I also don't understand why this lives in percpu.c...
> 
> I didn't think it called for adding another C file for this. How about 
> we
> rename this file to smp.c? Would that make sense for both?

Make that hyp-smp.c, please!

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