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

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Date:   Tue, 13 Dec 2022 16:07:50 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     David Matlack <dmatlack@...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 Fri, Dec 9, 2022 at 2:22 PM David Matlack <dmatlack@...gle.com> wrote:
>
> 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.

Right, good point. I'll fix that in the next edition.