Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings

[Ard Biesheuvel <ardb@kernel.org>]

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;
}