Re: [RFC PATCH v4 5/6] arm64: tlb: Use translation level hint in vm_flags

[Zhenyu Ye]

Hi Marc,

Thanks for your review.

On 2020/3/24 22:45, Marc Zyngier wrote:
> On Tue, 24 Mar 2020 21:45:33 +0800
> Zhenyu Ye <yezhenyu2@xxxxxxxxxx> wrote:
> 
>> This patch used the VM_LEVEL flags in vma->vm_flags to set the
>> TTL field in tlbi instruction.
>>
>> Signed-off-by: Zhenyu Ye <yezhenyu2@xxxxxxxxxx>
>> ---
>>  arch/arm64/include/asm/mmu.h      |  2 ++
>>  arch/arm64/include/asm/tlbflush.h | 14 ++++++++------
>>  arch/arm64/mm/mmu.c               | 14 ++++++++++++++
>>  3 files changed, 24 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
>> index d79ce6df9e12..a8b8824a7405 100644
>> --- a/arch/arm64/include/asm/mmu.h
>> +++ b/arch/arm64/include/asm/mmu.h
>> @@ -86,6 +86,8 @@ extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
>>  extern void *fixmap_remap_fdt(phys_addr_t dt_phys, int *size, pgprot_t prot);
>>  extern void mark_linear_text_alias_ro(void);
>>  extern bool kaslr_requires_kpti(void);
>> +extern unsigned int get_vma_level(struct vm_area_struct *vma);
>> +
>>  
>>  #define INIT_MM_CONTEXT(name)	\
>>  	.pgd = init_pg_dir,
>> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
>> index d141c080e494..93bb09fdfafd 100644
>> --- a/arch/arm64/include/asm/tlbflush.h
>> +++ b/arch/arm64/include/asm/tlbflush.h
>> @@ -218,10 +218,11 @@ static inline void flush_tlb_page_nosync(struct vm_area_struct *vma,
>>  					 unsigned long uaddr)
>>  {
>>  	unsigned long addr = __TLBI_VADDR(uaddr, ASID(vma->vm_mm));
>> +	unsigned int level = get_vma_level(vma);
>>  
>>  	dsb(ishst);
>> -	__tlbi_level(vale1is, addr, 0);
>> -	__tlbi_user_level(vale1is, addr, 0);
>> +	__tlbi_level(vale1is, addr, level);
>> +	__tlbi_user_level(vale1is, addr, level);
>>  }
>>  
>>  static inline void flush_tlb_page(struct vm_area_struct *vma,
>> @@ -242,6 +243,7 @@ static inline void __flush_tlb_range(struct vm_area_struct *vma,
>>  				     unsigned long stride, bool last_level)
>>  {
>>  	unsigned long asid = ASID(vma->vm_mm);
>> +	unsigned int level = get_vma_level(vma);
>>  	unsigned long addr;
>>  
>>  	start = round_down(start, stride);
>> @@ -261,11 +263,11 @@ static inline void __flush_tlb_range(struct vm_area_struct *vma,
>>  	dsb(ishst);
>>  	for (addr = start; addr < end; addr += stride) {
>>  		if (last_level) {
>> -			__tlbi_level(vale1is, addr, 0);
>> -			__tlbi_user_level(vale1is, addr, 0);
>> +			__tlbi_level(vale1is, addr, level);
>> +			__tlbi_user_level(vale1is, addr, level);
>>  		} else {
>> -			__tlbi_level(vae1is, addr, 0);
>> -			__tlbi_user_level(vae1is, addr, 0);
>> +			__tlbi_level(vae1is, addr, level);
>> +			__tlbi_user_level(vae1is, addr, level);
>>  		}
>>  	}
>>  	dsb(ish);
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 128f70852bf3..e6a1221cd86b 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -60,6 +60,20 @@ static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
>>  
>>  static DEFINE_SPINLOCK(swapper_pgdir_lock);
>>  
>> +inline unsigned int get_vma_level(struct vm_area_struct *vma)
>> +{
>> +	unsigned int level = 0;
>> +	if (vma->vm_flags & VM_LEVEL_PUD)
>> +		level = 1;
>> +	else if (vma->vm_flags & VM_LEVEL_PMD)
>> +		level = 2;
>> +	else if (vma->vm_flags & VM_LEVEL_PTE)
>> +		level = 3;
>> +
>> +	vma->vm_flags &= ~(VM_LEVEL_PUD | VM_LEVEL_PMD | VM_LEVEL_PTE);
>> +	return level;
>> +}
>> +
>>  void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
>>  {
>>  	pgd_t *fixmap_pgdp;
> 
> 
> If feels bizarre a TLBI is now a destructive operation: you've lost the
> flags by having cleared them. Even if that's not really a problem in
> practice (you issue TLBI because you've unmapped the VMA), it remains
> that the act of invalidating TLBs isn't expected to change a kernel
> structure (and I'm not even thinking about potential races here).

I cleared vm_flags here just out of caution, because every TLBI flush
action should set vm_flags themself. As I know, the TLB_LEVEL of an vma
will only be changed by transparent hugepage collapse and merge when
the page is not mapped, so there may not have potential races.

But you are right that TLBI should't change a kernel structure.
I will remove the clear action and add some notices here.

> 
> If anything, I feel this should be based around the mmu_gather
> structure, which already tracks the right level of information and
> additionally knows about the P4D level which is missing in your patches
> (even if arm64 is so far limited to 4 levels).
> 
> Thanks,
> 
> 	M.
> 

mmu_gather structure has tracked the level information, but we can only
use the info in the tlb_flush interface. If we want to use the info in
flush_tlb_range, we may should have to add a parameter to this interface,
such as:

	flush_tlb_range(vma, start, end, flags);

However, the flush_tlb_range is a common interface to all architectures,
I'm not sure if this is feasible because this feature is only supported
by ARM64 currently.

Or can we ignore the flush_tlb_range and only set the TTL field in
tlb_flush?  Waiting for your suggestion...


For P4D level, the TTL field is limited to 4 bit(2 for translation granule
and 2 for page table level), so we can no longer afford more levels :).