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

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Message-ID: <b39cb015-39bb-cb03-e095-414e323fd6aa@redhat.com>
Date:   Fri, 23 Apr 2021 10:42:08 +1000
From:   Gavin Shan <gshan@...hat.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Keqian Zhu <zhukeqian1@...wei.com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, wanghaibin.wang@...wei.com
Subject: Re: [PATCH v4 2/2] kvm/arm64: Try stage2 block mapping for host
 device MMIO

Hi Marc,

On 4/22/21 4:51 PM, Marc Zyngier wrote:
> On Thu, 22 Apr 2021 03:25:23 +0100,
> Gavin Shan <gshan@...hat.com> wrote:
>> On 4/21/21 4:36 PM, Keqian Zhu wrote:
>>> On 2021/4/21 15:52, Gavin Shan wrote:
>>>> 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@...wei.com>
>>>>> ---
>>>>>     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.
>>>
>>
>> I think I didn't make things clear. What I meant is VM_PFNMAP can't
>> be set for hugetlbfs VMAs. So the checks here can be simplified as
>> below if you agree:
>>
>>      if (is_vm_hugetlb_page(vma))
>>          return huge_page_shift(hstate_vma(vma));
>>
>>      if (!(vma->vm_flags & VM_PFNMAP))
>>          return PAGE_SHIFT;
>>
>>      VM_BUG_ON(is_vm_hugetlb_page(vma));       /* Can be dropped */
> 
> No. If this case happens, I want to see it. I have explicitly asked
> for it, and this check stays.
> 

Thanks for the explanation. To keep VM_BUG_ON() sounds good to me too :)

Thanks,
Gavin