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

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Date:   Tue, 1 Jun 2021 15:48:06 +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 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.

> > +
> > +	__list_del_entry(node);
> > +	memset(node, 0, sizeof(*node));
> > +}
> > +
> > +static inline void page_add_to_list(struct hyp_page *p, struct list_head *head)
> > +{
> > +	struct list_head *node = (struct list_head *)hyp_page_to_virt(p);
> > +
> > +	INIT_LIST_HEAD(node);
> > +	list_add_tail(node, head);
> > +}
> > +
> > +static inline struct hyp_page *node_to_page(struct list_head *node)
> > +{
> > +	return (struct hyp_page *)hyp_virt_to_page(node);
> 
> Why is this cast necessary? If I'm not mistaken, hyp_vmemmap is
> already cast as a 'struct hyp_page *', so hyp_virt_to_page() should
> return the same type.

Right, that one is totally unnecessary, I'll remove.

Cheers,
Quentin