linux-kernel - Re: [PATCH v5 19/25] arm64/mm: Wire up PTE

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Date: Tue, 13 Feb 2024 14:21:07 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Ard Biesheuvel <ardb@...nel.org>, David Hildenbrand <david@...hat.com>
Cc: Mark Rutland <mark.rutland@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
 Andrey Ryabinin <ryabinin.a.a@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, John Hubbard
 <jhubbard@...dia.com>, Zi Yan <ziy@...dia.com>,
 Barry Song <21cnbao@...il.com>, Alistair Popple <apopple@...dia.com>,
 Yang Shi <shy828301@...il.com>, Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, linux-arm-kernel@...ts.infradead.org,
 x86@...nel.org, linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings

On 13/02/2024 14:08, Ard Biesheuvel wrote:
> On Tue, 13 Feb 2024 at 15:05, David Hildenbrand <david@...hat.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@....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;
> }

Ahh, got it. Yes, I'll do it like this. Thanks!