[PATCH] arm64: mm: Align PGDs to at least 64 bytes

[Anshuman Khandual]

On 11/22/22 22:26, Ard Biesheuvel wrote:
> My copy of the ARM ARM (DDI 0487G.a) no longer describes the 64 byte
> minimum alignment of root page tables as being conditional on whether
> 52-bit physical addressing is supported and enabled, even though I seem
> to remember that this was the case formerly (and our code suggests the
> same).
ARM ARM (DDI 0487G.a) says,

"The minimum alignment of a translation table containing fewer than eight
entries is 64 bytes." But that does not seem to be conditional on 52-bit
physical address support, unless only the 52 bit physical address space
support could produce table configurations, which will have fewer than 8
entries in the PGD level.

> 
> So align pgd_t[] allocations to 64 bytes. Note that this only affects
> 16k/4 levels configurations, which are unlikely to be in wide use.

Instead of "16k/4 levels configurations", could we mention the in terms
of [PG_SIZE/VA_BITS/PA_BITS] format which can be easily related with
available config options.

> 
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  arch/arm64/mm/pgd.c | 13 +++----------
>  1 file changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm64/mm/pgd.c b/arch/arm64/mm/pgd.c
> index 4a64089e5771c1e2..8f01a75c35caaa9a 100644
> --- a/arch/arm64/mm/pgd.c
> +++ b/arch/arm64/mm/pgd.c
> @@ -40,17 +40,10 @@ void __init pgtable_cache_init(void)
>  	if (PGD_SIZE == PAGE_SIZE)
>  		return;
>  
> -#ifdef CONFIG_ARM64_PA_BITS_52
>  	/*
> -	 * With 52-bit physical addresses, the architecture requires the
> -	 * top-level table to be aligned to at least 64 bytes.
> +	 * Naturally aligned pgds required by the architecture, with a minimum
> +	 * alignment of 64 bytes.
>  	 */
> -	BUILD_BUG_ON(PGD_SIZE < 64);
> -#endif
> -
> -	/*
> -	 * Naturally aligned pgds required by the architecture.
> -	 */
> -	pgd_cache = kmem_cache_create("pgd_cache", PGD_SIZE, PGD_SIZE,
> +	pgd_cache = kmem_cache_create("pgd_cache", PGD_SIZE, max(PGD_SIZE, 64),
>  				      SLAB_PANIC, NULL);

There is a build warning as follows, which can be fixed with typecasting
constant 64 with (size_t). While here, it would also make sense to define
a macro for PGD minimum alignment requirement i.e 64 bytes.

diff --git a/arch/arm64/mm/pgd.c b/arch/arm64/mm/pgd.c
index 8f01a75c35ca..8d4b28d9590f 100644
--- a/arch/arm64/mm/pgd.c
+++ b/arch/arm64/mm/pgd.c
@@ -44,6 +44,6 @@ void __init pgtable_cache_init(void)
         * Naturally aligned pgds required by the architecture, with a minimum
         * alignment of 64 bytes.
         */
-       pgd_cache = kmem_cache_create("pgd_cache", PGD_SIZE, max(PGD_SIZE, 64),
+       pgd_cache = kmem_cache_create("pgd_cache", PGD_SIZE, max(PGD_SIZE, (size_t) 64),
                                      SLAB_PANIC, NULL);
 }

In file included from ./include/linux/kernel.h:26,                                                                                       
                 from ./arch/arm64/include/asm/cpufeature.h:22,                                                                          
                 from ./arch/arm64/include/asm/ptrace.h:11,
                 from ./arch/arm64/include/asm/irqflags.h:10,
                 from ./include/linux/irqflags.h:16,                                                                                                                                                                                                                              
                 from ./include/linux/spinlock.h:59,                
                 from ./include/linux/mmzone.h:8,                   
                 from ./include/linux/gfp.h:7,                                                                                           
                 from ./include/linux/mm.h:7,
                 from arch/arm64/mm/pgd.c:9:
arch/arm64/mm/pgd.c: In function ‘pgtable_cache_init’:                                                                                   
./include/linux/minmax.h:20:28: warning: comparison of distinct pointer types lacks a cast                                               
   20 |  (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1))) 
      |                            ^~                                                                                                                                                                                                                                             
./include/linux/minmax.h:26:4: note: in expansion of macro ‘__typecheck’                                                                                                                                                                                                          
   26 |   (__typecheck(x, y) && __no_side_effects(x, y))                                                                                                                                                                                                                          
      |    ^~~~~~~~~~~                             
./include/linux/minmax.h:36:24: note: in expansion of macro ‘__safe_cmp’                                                                                                                                                                                                          
   36 |  __builtin_choose_expr(__safe_cmp(x, y), \                                                                                                                                                                                                                                
      |                        ^~~~~~~~~~                                                                                                                                                                                                                                         
./include/linux/minmax.h:52:19: note: in expansion of macro ‘__careful_cmp’                                                                                                                                                                                                       
   52 | #define max(x, y) __careful_cmp(x, y, >)                                                                                                                                                                                                                                  
      |                   ^~~~~~~~~~~~~                                                                                                                                                                                                                                           
arch/arm64/mm/pgd.c:47:55: note: in expansion of macro ‘max’                                                                                                                                                                                                                      
   47 |  pgd_cache = kmem_cache_create("pgd_cache", PGD_SIZE, max(PGD_SIZE, 64),  

>  }

Besides, tested on config [16K|48VA|48PA] producing "CONFIG_PGTABLE_LEVELS=4"
and with PGD_SIZE != PAGE_SIZE, which booted successfully.