linux-kernel - Re: [PATCH 1/7] KVM: x86/MMU: Move pte

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Message-ID: <Y5O1LbbeI7XXeaT2@google.com>
Date:   Fri, 9 Dec 2022 14:22:37 -0800
From:   David Matlack <dmatlack@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vipin Sharma <vipinsh@...gle.com>
Subject: Re: [PATCH 1/7] KVM: x86/MMU: Move pte_list operations to rmap.c

On Tue, Dec 06, 2022 at 05:35:55PM +0000, Ben Gardon wrote:
> In the interest of eventually splitting the Shadow MMU out of mmu.c,
> start by moving some of the operations for manipulating pte_lists out of
> mmu.c and into a new pair of files: rmap.c and rmap.h.
> 
> No functional change intended.
> 
> Signed-off-by: Ben Gardon <bgardon@...gle.com>
> ---
[...]
> diff --git a/arch/x86/kvm/mmu/rmap.h b/arch/x86/kvm/mmu/rmap.h
> new file mode 100644
> index 000000000000..059765b6e066
> --- /dev/null
> +++ b/arch/x86/kvm/mmu/rmap.h
> @@ -0,0 +1,34 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#ifndef __KVM_X86_MMU_RMAP_H
> +#define __KVM_X86_MMU_RMAP_H
> +
> +#include <linux/kvm_host.h>
> +
> +/* make pte_list_desc fit well in cache lines */
> +#define PTE_LIST_EXT 14
> +
> +/*
> + * Slight optimization of cacheline layout, by putting `more' and `spte_count'
> + * at the start; then accessing it will only use one single cacheline for
> + * either full (entries==PTE_LIST_EXT) case or entries<=6.
> + */
> +struct pte_list_desc {
> +	struct pte_list_desc *more;
> +	/*
> +	 * Stores number of entries stored in the pte_list_desc.  No need to be
> +	 * u64 but just for easier alignment.  When PTE_LIST_EXT, means full.
> +	 */
> +	u64 spte_count;
> +	u64 *sptes[PTE_LIST_EXT];
> +};
> +
> +static struct kmem_cache *pte_list_desc_cache;

The definition of pte_list_desc_cache needs to go in a C file since it's
a global variable. Since it now needs to be accessed by more than once C
file, drop the static. Then it can be accessed with extern.

Since most of the code that sets up and deals with pte_list_desc_cache
is still in mmu.c, my vote is to keep the definition there.

i.e.

mmu.c:

  struct kmem_cache *pte_list_desc_cache;

rmap.c

  extern struct kmem_cache *pte_list_desc_cache;

And no need for anything in rmap.h.