Re: [PATCH v4 2/2] kvm/arm64: Try stage2 block mapping for host device MMIO

[Keqian Zhu]

On 2021/4/21 15:52, Gavin Shan wrote:
> Hi Keqian,
> 
> On 4/16/21 12:03 AM, Keqian Zhu wrote:
>> The MMIO region of a device maybe huge (GB level), try to use
>> block mapping in stage2 to speedup both map and unmap.
>>
>> Compared to normal memory mapping, we should consider two more
>> points when try block mapping for MMIO region:
>>
>> 1. For normal memory mapping, the PA(host physical address) and
>> HVA have same alignment within PUD_SIZE or PMD_SIZE when we use
>> the HVA to request hugepage, so we don't need to consider PA
>> alignment when verifing block mapping. But for device memory
>> mapping, the PA and HVA may have different alignment.
>>
>> 2. For normal memory mapping, we are sure hugepage size properly
>> fit into vma, so we don't check whether the mapping size exceeds
>> the boundary of vma. But for device memory mapping, we should pay
>> attention to this.
>>
>> This adds get_vma_page_shift() to get page shift for both normal
>> memory and device MMIO region, and check these two points when
>> selecting block mapping size for MMIO region.
>>
>> Signed-off-by: Keqian Zhu <zhukeqian1@xxxxxxxxxx>
>> ---
>>   arch/arm64/kvm/mmu.c | 61 ++++++++++++++++++++++++++++++++++++--------
>>   1 file changed, 51 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index c59af5ca01b0..5a1cc7751e6d 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -738,6 +738,35 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot,
>>       return PAGE_SIZE;
>>   }
>>   +static int get_vma_page_shift(struct vm_area_struct *vma, unsigned long hva)
>> +{
>> +    unsigned long pa;
>> +
>> +    if (is_vm_hugetlb_page(vma) && !(vma->vm_flags & VM_PFNMAP))
>> +        return huge_page_shift(hstate_vma(vma));
>> +
>> +    if (!(vma->vm_flags & VM_PFNMAP))
>> +        return PAGE_SHIFT;
>> +
>> +    VM_BUG_ON(is_vm_hugetlb_page(vma));
>> +
> 
> I don't understand how VM_PFNMAP is set for hugetlbfs related vma.
> I think they are exclusive, meaning the flag is never set for
> hugetlbfs vma. If it's true, VM_PFNMAP needn't be checked on hugetlbfs
> vma and the VM_BUG_ON() becomes unnecessary.
Yes, but we're not sure all drivers follow this rule. Add a BUG_ON() is
a way to catch issue.

> 
>> +    pa = (vma->vm_pgoff << PAGE_SHIFT) + (hva - vma->vm_start);
>> +
>> +#ifndef __PAGETABLE_PMD_FOLDED
>> +    if ((hva & (PUD_SIZE - 1)) == (pa & (PUD_SIZE - 1)) &&
>> +        ALIGN_DOWN(hva, PUD_SIZE) >= vma->vm_start &&
>> +        ALIGN(hva, PUD_SIZE) <= vma->vm_end)
>> +        return PUD_SHIFT;
>> +#endif
>> +
>> +    if ((hva & (PMD_SIZE - 1)) == (pa & (PMD_SIZE - 1)) &&
>> +        ALIGN_DOWN(hva, PMD_SIZE) >= vma->vm_start &&
>> +        ALIGN(hva, PMD_SIZE) <= vma->vm_end)
>> +        return PMD_SHIFT;
>> +
>> +    return PAGE_SHIFT;
>> +}
>> +
> 
> There is "switch(...)" fallback mechanism in user_mem_abort(). PUD_SIZE/PMD_SIZE
> can be downgraded accordingly if the addresses fails in the alignment check
> by fault_supports_stage2_huge_mapping(). I think it would make user_mem_abort()
> simplified if the logic can be moved to get_vma_page_shift().
> 
> Another question if we need the check from fault_supports_stage2_huge_mapping()
> if VM_PFNMAP area is going to be covered by block mapping. If so, the "switch(...)"
> fallback mechanism needs to be part of get_vma_page_shift().
Yes, Good suggestion. My idea is that we can keep this series simpler and do further
optimization in another patch series. Do you mind to send a patch?

Thanks,
Keqian