linux-kernel - Re: [PATCH 1/1] KVM: arm64: Use a separate function for hyp stage-1 walks

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Date:   Tue, 15 Nov 2022 17:23:51 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Will Deacon <will@...nel.org>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] KVM: arm64: Use a separate function for hyp stage-1
 walks

Hey Will,

On Tue, Nov 15, 2022 at 01:25:34PM +0000, Will Deacon wrote:

[...]

> On Mon, Nov 14, 2022 at 08:11:27PM +0000, Oliver Upton wrote:
> > +int kvm_pgtable_hyp_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
> > +			 struct kvm_pgtable_walker *walker);
> 
> Hmm, this feels like slightly the wrong abstraction to me -- there's nothing
> hyp-specific about the problem being solved, it's just that the only user
> is for hyp walks.
> 
> Could we instead rework 'struct kvm_pgtable' slightly so that the existing
> 'flags' field is no-longer stage-2 specific and includes a KVM_PGTABLE_LOCKED
> flag which could be set by kvm_pgtable_hyp_init()?
> 
> That way the top-level API remains unchanged and the existing callers will
> continue to work.

Thanks for the suggestion! Yeah, this should be described by the flags
instead.

We already have KVM_PGTABLE_WALK_SHARED, I could actually condition the
RCU lock/unlock on that one. That would make it an explicit opt-in
instead of requiring an opt out with callers passing KVM_PGTABLE_LOCKED.

Thoughts?

--
Thanks,
Oliver