linux-kernel - Re: [PATCH v7 2/8] KVM: Introduce __kvm_follow

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Date:   Thu, 6 Jul 2023 14:52:55 +0900
From:   David Stevens <stevensd@...omium.org>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Xu <peterx@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v7 2/8] KVM: Introduce __kvm_follow_pfn function

On Thu, Jul 6, 2023 at 10:34 AM Isaku Yamahata <isaku.yamahata@...il.com> wrote:
>
> On Tue, Jul 04, 2023 at 04:50:47PM +0900,
> David Stevens <stevensd@...omium.org> wrote:
>
> > From: David Stevens <stevensd@...omium.org>
> >
> > Introduce __kvm_follow_pfn, which will replace __gfn_to_pfn_memslot.
> > __kvm_follow_pfn refactors the old API's arguments into a struct and,
> > where possible, combines the boolean arguments into a single flags
> > argument.
> >
> > Signed-off-by: David Stevens <stevensd@...omium.org>
> > ---
> >  include/linux/kvm_host.h |  16 ++++
> >  virt/kvm/kvm_main.c      | 171 ++++++++++++++++++++++-----------------
> >  virt/kvm/kvm_mm.h        |   3 +-
> >  virt/kvm/pfncache.c      |   8 +-
> >  4 files changed, 122 insertions(+), 76 deletions(-)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 9d3ac7720da9..ef2763c2b12e 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -97,6 +97,7 @@
> >  #define KVM_PFN_ERR_HWPOISON (KVM_PFN_ERR_MASK + 1)
> >  #define KVM_PFN_ERR_RO_FAULT (KVM_PFN_ERR_MASK + 2)
> >  #define KVM_PFN_ERR_SIGPENDING       (KVM_PFN_ERR_MASK + 3)
> > +#define KVM_PFN_ERR_NEEDS_IO (KVM_PFN_ERR_MASK + 4)
> >
> >  /*
> >   * error pfns indicate that the gfn is in slot but faild to
> > @@ -1156,6 +1157,21 @@ unsigned long gfn_to_hva_memslot_prot(struct kvm_memory_slot *slot, gfn_t gfn,
> >  void kvm_release_page_clean(struct page *page);
> >  void kvm_release_page_dirty(struct page *page);
> >
> > +struct kvm_follow_pfn {
> > +     const struct kvm_memory_slot *slot;
> > +     gfn_t gfn;
> > +     unsigned int flags;
> > +     bool atomic;
> > +     /* Allow a read fault to create a writeable mapping. */
> > +     bool allow_write_mapping;
>
> Maybe, make them const for input arguments?

Unfortunately using const isn't straightforward as long as the kernel
continues to use -Wdeclaration-after-statement. If these fields were
const, then they would need to be specified in the initializer when
declaring the variable, but that's not necessarily always possible.

-David