linux-kernel - Re: [PATCH 3/7] KVM: arm64: Remove list_head from hyp

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Date:   Wed, 2 Jun 2021 09:23:33 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     will@...nel.org, james.morse@....com, alexandru.elisei@....com,
        catalin.marinas@....com, suzuki.poulose@....com,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kernel-team@...roid.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] KVM: arm64: Remove list_head from hyp_page

On Tuesday 01 Jun 2021 at 18:41:49 (+0100), Marc Zyngier wrote:
> On Tue, 01 Jun 2021 16:48:06 +0100,
> Quentin Perret <qperret@...gle.com> wrote:
> > 
> > On Tuesday 01 Jun 2021 at 15:38:22 (+0100), Marc Zyngier wrote:
> > > On Thu, 27 May 2021 13:51:30 +0100,
> > > Quentin Perret <qperret@...gle.com> wrote:
> > > > +/*
> > > > + * Pages that are available for allocation are tracked in free-lists, so we use
> > > > + * the pages themselves to store the list nodes to avoid wasting space. As the
> > > > + * allocator always returns zeroed pages (which are zeroed on the hyp_put_page()
> > > > + * path to optimize allocation speed), we also need to clean-up the list node in
> > > > + * each page when we take it out of the list.
> > > > + */
> > > > +static inline void page_remove_from_list(struct hyp_page *p)
> > > > +{
> > > > +	struct list_head *node = (struct list_head *)hyp_page_to_virt(p);
> > > 
> > > Nit: How about changing hyp_page_to_virt() so that it returns a
> > > convenient 'void *', and get rid of the ugly casts?
> > 
> > It should already return void *, but I kind of liked the explicit cast
> > here for documentation purpose. We're turning a 'random' piece of unused
> > memory into a typed object, so that felt like a useful annotation. Happy
> > to get rid of it though.
> 
> My expectations were that using hyp_page_to_virt() already serves as a
> pretty big warning that we're doing something unusual.
> 
> I guess that if we want to be really careful about those, we should
> then be consistent and make it return a uintptr_t (or unsigned long)
> instead, actively making use of the cast, consistently, everywhere.

Right, so I'd prefer keeping it void * for consistency with the EL1
equivalent -- much of this aims to be API compatible with the Linux
stuff. I'll get rid of the cast and post a v2 shortly.

Cheers,
Quentin