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From: Ard Biesheuvel <ardb@kernel.org>
Date: Tue, 13 Feb 2024 15:08:39 +0100
Message-ID: <CAMj1kXEK6wiHkO9sJ5h6D3L88dR=2UndEqGeQjv=TftZ3PnN9w@mail.gmail.com>
Subject: Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings
To: David Hildenbrand <david@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>, Mark Rutland <mark.rutland@arm.com>, 
	Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>, 
	James Morse <james.morse@arm.com>, Andrey Ryabinin <ryabinin.a.a@gmail.com>, 
	Andrew Morton <akpm@linux-foundation.org>, Matthew Wilcox <willy@infradead.org>, 
	Kefeng Wang <wangkefeng.wang@huawei.com>, John Hubbard <jhubbard@nvidia.com>, 
	Zi Yan <ziy@nvidia.com>, Barry Song <21cnbao@gmail.com>, Alistair Popple <apopple@nvidia.com>, 
	Yang Shi <shy828301@gmail.com>, Nicholas Piggin <npiggin@gmail.com>, 
	Christophe Leroy <christophe.leroy@csgroup.eu>, "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>, 
	"Naveen N. Rao" <naveen.n.rao@linux.ibm.com>, Thomas Gleixner <tglx@linutronix.de>, 
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, "H. Peter Anvin" <hpa@zytor.com>, 
	linux-arm-kernel@lists.infradead.org, x86@kernel.org, 
	linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, 
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On Tue, 13 Feb 2024 at 15:05, David Hildenbrand <david@redhat.com> wrote:
>
> On 13.02.24 15:02, Ryan Roberts wrote:
> > On 13/02/2024 13:45, David Hildenbrand wrote:
> >> On 13.02.24 14:33, Ard Biesheuvel wrote:
> >>> On Tue, 13 Feb 2024 at 14:21, Ryan Roberts <ryan.roberts@arm.com> wrote:
> >>>>
> >>>> On 13/02/2024 13:13, David Hildenbrand wrote:
..
> >>>>> Just a thought, you could have a is_efi_mm() function that abstracts all that.
> >>>>>
> >>>>> diff --git a/include/linux/efi.h b/include/linux/efi.h
> >>>>> index c74f47711f0b..152f5fa66a2a 100644
> >>>>> --- a/include/linux/efi.h
> >>>>> +++ b/include/linux/efi.h
> >>>>> @@ -692,6 +692,15 @@ extern struct efi {
> >>>>>
> >>>>>    extern struct mm_struct efi_mm;
> >>>>>
> >>>>> +static inline void is_efi_mm(struct mm_struct *mm)
> >>>>> +{
> >>>>> +#ifdef CONFIG_EFI
> >>>>> +       return mm == &efi_mm;
> >>>>> +#else
> >>>>> +       return false;
> >>>>> +#endif
> >>>>> +}
> >>>>> +
> >>>>>    static inline int
> >>>>>    efi_guidcmp (efi_guid_t left, efi_guid_t right)
> >>>>>    {
> >>>>>
> >>>>>
> >>>>
> >>>> That would definitely work, but in that case, I might as well just check for it
> >>>> in mm_is_user() (and personally I would change the name to mm_is_efi()):
> >>>>
> >>>>
> >>>> static inline bool mm_is_user(struct mm_struct *mm)
> >>>> {
> >>>>           return mm != &init_mm && !mm_is_efi(mm);
> >>>> }
> >>>>
> >>>> Any objections?
> >>>>
> >>>
> >>> Any reason not to use IS_ENABLED(CONFIG_EFI) in the above? The extern
> >>> declaration is visible to the compiler, and any references should
> >>> disappear before the linker could notice that efi_mm does not exist.
> >>>
> >>
> >> Sure, as long as the linker is happy why not. I'll let Ryan mess with that :)
> >
> > I'm not sure if you are suggesting dropping the mm_is_efi() helper and just use
> > IS_ENABLED(CONFIG_EFI) in mm_is_user() to guard efi_mm, or if you are suggesting
> > using IS_ENABLED(CONFIG_EFI) in mm_is_efi() instead of the ifdefery?
> >
> > The former was what I did initially; It works great, but I didn't like that I
> > was introducing a new code dependecy between efi and arm64 (nothing else outside
> > of efi references efi_mm).
> >
> > So then concluded that it is safe to not worry about efi_mm (thanks for your
> > confirmation). But then David wanted a VM_WARN check, which reintroduces the
> > code dependency. So he suggested the mm_is_efi() helper to hide that... This is
> > all starting to feel circular...
>
> I think Ard meant that inside mm_is_efi(), we could avoid the #ifdef and
> simply use IS_ENABLED().
>

Yes.

static inline void mm_is_efi(struct mm_struct *mm)
{
    return IS_ENABLED(CONFIG_EFI) && mm == &efi_mm;
}