Re: [PATCH 3/5] KVM: arm64: Only return attributes from stage2_update_leaf_attrs()

From: Marc Zyngier <maz@kernel.org>
To: Oliver Upton <oliver.upton@linux.dev>
Cc: James Morse <james.morse@arm.com>,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	Quentin Perret <qperret@google.com>,
	Will Deacon <will@kernel.org>, Reiji Watanabe <reijiw@google.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>
Subject: Re: [PATCH 3/5] KVM: arm64: Only return attributes from stage2_update_leaf_attrs()
Date: Tue, 07 Feb 2023 14:56:49 +0000	[thread overview]
Message-ID: <86r0v1zfpq.wl-maz@kernel.org> (raw)
In-Reply-To: <Y9w0ZYVXRgpTKuFD@google.com>

On Thu, 02 Feb 2023 22:08:37 +0000,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> On Wed, Jan 11, 2023 at 05:21:13PM +0000, Oliver Upton wrote:
> > Hey Marc,
> > 
> > On Wed, Jan 11, 2023 at 08:52:25AM +0000, Marc Zyngier wrote:
> > > [+ Suzuki and Zenghui who are missing from the Cc list]
> > 
> > Doh! Just switched over to working out of a new git tree and didn't move
> > over my cc-cmd. Apologies to you two.
> > 
> > > On Wed, 11 Jan 2023 00:02:58 +0000,
> > > Oliver Upton <oliver.upton@linux.dev> wrote:
> > > > 
> > > > Returning a single PTE from stage2_update_leaf_attrs() doesn't make a
> > > > great deal of sense given that the function could be used to apply a
> > > > change to a range of PTEs. Instead, return a bitwise OR of attributes
> > > > from all the visited PTEs.
> > > 
> > > I find this amalgamation of attributes quite confusing, and I have a
> > > hard time attaching semantics to the resulting collection of bits.
> > > 
> > > It also means that you cannot reason about a particular attribute
> > > being 0 if any of the neighbour PTEs has this bit set.
> > 
> > Very true. What I had really wanted to do was make a walker that allows
> > software to check the state of specific attribute bit(s) within a range
> > of memory instead of returning all of them. I decided against it because
> > it would put more churn on other callers or require a new walker
> > entirely.
> > 
> > Anyway, I can go about that change to make this a bit easier to reason
> > about. Thoughts?
> 
> LOL, I thought I hadn't replied which is why I didn't hear anything
> back. Ball is in your court, Marc, any thoughts?

Huh, I clearly missed that email. Sorry about the delay.

Having a separate walker doesn't strike me as unreasonable. This
clearly is something different from the existing walkers, and it is
bound to be very little code anyway.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.