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

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Date:   Wed, 11 Nov 2020 13:52:06 +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 13:45, David Brazdil wrote:
> On Wed, Nov 11, 2020 at 01:29:29PM +0000, Marc Zyngier wrote:
>> 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.
> 
> EL2 can't, at least not accidentally. The hyp memory mapping is 
> PAGE_HYP_RO
> (see patch 05).

Ah, I obviously overlooked that. Thanks for setting me straight.

> Shouldn't clash with include files. Where fixing the kernel might clash 
> is
> all the users of for_each_*_cpu that use an int for the iterator var.

I don't think that's a problem (nobody expects that many CPUs). But if 
you
are confident that we don't have a problem, no need to change the kernel
itself.

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